Can some I'd this please

[wizardsrelm]

I found this and 2 other on a colony rock I bought 5 months ago in the past 2 months it became this any ideas would be helpful I know the pictures are not great but hopefully good enough it's about 1 1/2" long and about 3/4 wide
Thanks

 

[Tahoe61]

Yup not sure what I am looking at. ;Bored Is it algae or animal? Could it be a Mussel?

 

[wizardsrelm]

It's pretty transparent except for the black line in it

 

[jsker]

With out a clearer picture to be sure but @Tahoe61 seems to be spot on.

 

[wizardsrelm]

Thanks guys