Possibly, depending on interpretation of prerequisites
On feats, the PHB says:
You must meet any prerequisite specified in a feat to take that feat. If you ever lose a feat’s prerequisite, you can’t use that feat until you regain the prerequisite.
So our first question is whether we can meet the prerequisites for Elven Accuracy while we are shapechanged. The prerequisite for Elven Accuracy is "Elf or half-elf" (XGtE 74). This is the full text; no explanation is provided. So how are we to interpret this phrase - your race upon character creation must be elf to later take the elven accuracy feat, or you must currently be in the body of an elf to meet the prerequisite of the feat?
Since Xanathar's does not define what they mean by a prerequisite, how does it describe the racial feats in general?
A racial feat represents [...] a deepening connection to your race’s culture.
It doesn't seem, on the face of it, that being shapechanged would lessen your connection to your culture, since the caster retains their Intelligence, Wisdom, alignment, and personality.
If we need to currently be an elf, we cannot use Elven Accuracy while shapechanged. But if all we need to use Elven Accuracy is is an authentic understanding of Elven culture, it is possible that the feat can be used while shapechanged.
What are the limitations of shapechange? As correctly cited by the querent:
You retain the benefit of any features from your class, race, or other source and can use them, provided that your new form is physically capable of doing so.
Feats, including Elven Accuracy, are benefits of a feature 'from another source'. Thus one part of our decision is whether the changed shape is physically capable of using the feat. Again, if all we need is an appreciation for elven culture, the changed shape should be physically capable of that.
Turning to the feat itself, just the small part quoted by the querent says:
Whenever you have advantage on an attack roll using Dexterity, Intelligence, Wisdom, or Charisma, you can reroll one of the dice once.
There are certainly plenty of changed shapes that will be making attack rolls using their Dexterity. However, this is not the only part of the feat. In particular, just as there is no flavor text in spells, there is no flavor text in feats. Examining the full description of the feat, we find:
The accuracy of elves is legendary, especially that of elf archers and spellcasters. You have uncanny aim with attacks that rely on precision rather than brute force.
This is not a prohibition against melee attacks just because archers and spellcasters are exemplars; of course an elf with Elven Accuracy is going to to be able to use it for melee finesse weapons as well. However, what we are trying to determine is the appropriateness of the feat for use with the attacks of changed shape, with our test case being a Marilith. Thus we need to consider, are there any attacks a Marilith could make that physically resemble those of elven warriors and spellcasters? That is, are they physically capable of attacking in such a way that they could benefit from a deeper connection to the attack forms celebrated in elven culture?
Dating back at least to first edition, D&D elves have been associated with swords, bows, and spells. 5e expresses this in Elf Weapon Training, a racial feature of High Elves. Regardless of class, High Elves have:
proficiency with the longsword, shortsword, shortbow, and longbow.
This is how I would understand the feat - elven culture places an emphasis on training in spells and certain weapon forms that favor accuracy over force, either delivering accurate missile fire, or darting in to deliver accurate thrusting and slashing blows with a long, light weapon and then backing out to parry. These attacks are made using a bipedal (Humanoid: Elf) body, with two limbs for movement and two hands holding weapons.
Now, a Marilith certainly does not have precisely this shape, but she does attack with weapon strikes made from a generally upright form. And while her stat block indicates that she is using her strength to make her longsword attacks, we note that her dexterity is actually higher than her strength, and that she has a natural parry feature for defense. Given this, I would be inclined to permit an Elf shapechanged into a Marilith to use the Elven Accuracy feat provided that such an Elf equipped themselves with Dex-based weapons (shortswords, bows) that they had proficiency in as an Elf and which had a plausible connection to Elven culture.
Two interpretations
We have two possible ways to understand the 'prerequisite' for using the feat. Either you must use the feat while you are in the body of an elf (and a shapechanged elf is not in such a body), or you must use it to make attacks in such a way that they rely on your deep connection to elven culture, especially like an archer or spellcaster, but including melee finesse weapons.
Obviously this is a rules interpretation, which is what the shapechange phrase 'physically capable' insists that we do, interpret. Most of my games are pretty narrativist and I would be very comfortable with the latter ruling in them. If I was running a character optimization game where backstory was optional and the concept of 'deepening their connection to elven culture' got me blank stares from the players, then I would let them use the feat with any attack that was clearly based on Dexterity, regardless of its connection to elven culture.
On the other hand, if I found myself DMing a game where a character had somehow equipped themselves with six vorpal scimitars and wanted to use Elven Accuracy as a Marilith to decapitate opponents right and left, invoking the 'you are not currently an elf' prerequisite to disallow it would not give me pause.
