Gray mat looking thing??

[BrandonV2193]

I bought some zoa's yesterday and noticed this gray mat like thing on the back of the rock they are on. It's smooth to the touch, not slimy but smooth. Haven't a clue what it is but it feels like it's living. Any ideas? Also this is literally what it looks like focused and zoomed in on. there aren't any identifying details about it.

 

[Bounce House]

Looks like maybe a sponge. You may just want to take the rock out and scrape it off with a brush just for peace of mind if you can.

 

[Poseidon03]

Looks like a sponge or melted zoa