what is this?

button polyp or a zoo

Looks like the arm of a brittle star to me, I've got a couple of those. Nothing to worry about, in fact nice to have. Don't expect to see any more of it than an arm sticking out.

if the arm just sticks out and does nothing i would say a star.. but i had something almost exactly in my LR and one night i saw it picking at the rock.. found out it was a worm.. but i wouldnt worry about it either way..