What’s on my snail?

[ASIN28]

Hey all,
Got some nassarius snails yesterday, scrubbed shells etc before putting them into the tank. I noticed this particular snail has this flat thing on his foot I guess you would call it. Maybe something just sticking to him or is this that flatworm?
Thanks best pic I could get.

 

[LeftyReefer]

Its part of the snail. I believe its called an operculum.

It basically acts as a door/seal when they are fully retracted into their shell.