I tried to pick up the rock and get a close up picture, but it retracted back a little and I could only get a picture of the top.
Didn't really want to bother it, but if a better picture is needed I could try getting it out.
It gets no more than an inch long when extended, it has a little hole in the middle of the top, surrounded by tentacles.
I saw it stuck to my overflow box and I thought it was trash because I set up my tank only 2 days ago and when i realized it was alive I left it on a rock.
If its not harmful, then it's really not a big deal knowing what it is, but it looks like some sort of echinoderm.