I agree, I often forget that corals are not plants(but they look like plants, which probably means that the Mars rock that look like a coral also looks like a plant).

And they act like plants: they photosynthesize; they are sessile; and they are structured as a thin layer of living tissue over a dead, rigid internal support.

I could see that, or a sea sponge[1]

1: https://study.com/academy/lesson/sea-sponge-lesson-for-kids....