ID help with this creature please?

[budsreef]

This creature has been in the exact same spot in my tank since I started it up about 6 weeks ago. It is about 1/8" to 3/16" in length, about half as wide and shaped like a hoof with an irregular split down the middle. It only comes out as far as this picture shows and is quick to pull in so that it becomes a flat surface with the rock exactly filling the hole it is in. I have never seen an eye, tentacle or any other kind of body part at all. The white spot that sort of looks like an eye seems to be the same as the white spots that are on the rock. The two halves seem to move independently in a very restricted motion but both halves come out and go in at the same time. Once I saw a very tiny hitchhiking nassarius looking snail crawl towards it and when it got close the creature just pulled in and let the snail crawl right across it. Any ideas?
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[george]

Some kind of hitchhiker bivalve from the sound of it. I'm not sure which part of the pic it's supposed to be, though.

 

[budsreef]

Hey George, thanks. It is the whitish blob right in the midlle of the picture.

 

[dakota]

Probably an oyster of some kind. I have a few that do the same thing. Cool.
Don't worry about it. They are good for your tank.

 

[budsreef]

Thanks Dakota, I didn't realize there were ones that don't have shells. I'll research that.