They are a mini Nem I have a couple they will move around if they do not like where they are and Peppermint shrimp will eat them. They are harmless basically a rock Nem they have a bit of color to them. Someone on here told me what they were called about 8 months ago when I posted a pic of one I have in my tank. Mine have not migrated much I believe I only have maybe 2 left as my peppermint shrimp ate a couple others. They don't seem to hurt anything I think they are eating some tiny snails at night as I have a few mini snails in my tank.

Bill