Hard to tell from pic but looks like a Boring Clam, if so just a good filter feeder and have many in my system. Mostly they are underneath or in crevices, some are even quite colorful with blues and greens.
Sounds silly but almost looks like it's got a set of teeth. Il look it up and try find out what it is. That piece of rock I driving me mad pulled 5 nudibranches off it recently and zoas won't open because of them