Shell near surface

[Royal_pudding]

Hi all
I have searched the forum and the interwebs looking for an answer. Im beginning to think I dont even know what to search for.
So I post here.

Today I found what looks to be a shell of some sort near the surface of my 5 week old tank.

It was cycled with wet liverock so Im pretty sure there are some hitchhikers. I spotted 2-3 aiptasia but the filefish took care of that.

Inhabitants
1 x aiptasia eating filefish
1 x blenny
2 x trochus snails
1 x hermit crab

Here are 2 pics from this thing in the tank and 1 from "it" on the table

 

[Glenner’sreef]

Looks harmless, but when in doubt, take it out!

 

[Rick's Reviews]

Reminds me of a film called 'cocoon'
looks weirdly like a snail shell that my hermits live in but they are black.
Yours is very white but unsual in an aquarium more like a moth or butterfly

 

[vetteguy53081]

Its an empty shell that has air in it and is floating.

 

[Royal_pudding]

Yes very weird, it seemed go be stuck to the back wall, no creature inside.