Pest ID and Solution needed please.

[Fellow]

Hey all.

I was wondering if anyone had some insight into what is slowly taking over my tank.

A few bad (sorry) pictures)





They range in diameter of a pencil eraser to larger than a nickel.

They are strictly nocturnal, when the lights are on they retract both their tentacles and body completely.

I have probably a dozen at least. I pretty sure came in with a chunk of rock my candy canes were attached to that I didn't QT which serves me right...

any help would be appreciated as I haven't found any pics of Apistia or Mojano that looks like what I have.