Mystery mollusk - ID help

[fatoldsun]

Any guess what I might have here – I tend to think these random forms of life that “pop” up are cool and I like to think they’re a “good” thing. Anyway, every time I find something and EVERYTIME I post it, turns out it’s a deadly killer something or other so I figured I’d start tring to figure out what this is – it’s really hard to get a clear pic - this is the best I could muster. The description which may make more sense – it looks like a small clam. If I look closely it has the “lines” in the “shell” that look like a common clam. Unlike clams I’m familiar with, this is on the underside of a rock and seems to be embedded in the rock – it’s grown out of a hole or pore in the rock. I’ve never noticed it open or filtering like a clam and I’ve never seen anything that looked like a foot – I’ve never seen it move either. So, any thoughts. Should I leave it alone or get it out?

 

[MadReefer]

Is hard or soft when you touch it?
From my experience I have seen something similar in my tank. It was white/yellow with some stripes and turned out to be some type of sponge.
Never caused any problems but cannot be certain from that pic.

 

[fatoldsun]

Haven't touched it yet. I'll have to check that out...

 

[TanksNStuff]

It's hard to see from the pic, but I think it looks sort of like something I've had growing in my sump for about a year now. Mine seems pretty harmless, but I've never seen it open either so not sure if it's beneficial at all. Maybe it opens at night or something?

If you stop over for that monti frag this week, you can take a peek and see if it looks like yours.

 

[NikkiT]

I wish I could ID it for you, but I can't see it clearly enough in that picture I am afraid.
That said, I do have hitchhiker molusks, one of which is a 2" bivalve that has resided in our 12g since early 2009 at about .2", so here's to hoping you finally have a good hitchhiker.