Grey Cyano?

[Walla2GSP]

Sorry, it's impossible to get a picture with where it is positioned in the tank, but I have something weird in my tank and I'm not sure what to make of it. My FOWLR tank was getting out of control and I've been wanting to add some dry rock. So, last week I re-aquascaped and added a couple more shelves and caves for the puffers. In the process, I disturbed two rocks encrusted with tubeworms. While putting them back I noticed a grey coating on the bottom of one that was the color of damp/dry concrete and sticky. This morning I noticed a small patch had spread to a neighboring rock. Anybody have any ideas? Ever heard of something like this?

[Walla2GSP]

http://i1268.photobucket.com/albums/jj568/jared_johnson8/20161023_172433_zpsbx2xebv1.jpg

This was the best shot i could get before I lost the lights, but the dark grey is not the natural color of the rock. I will try to get a better shot with the Canon tomorrow.

[rtparty]

Sponge?

[Walla2GSP]

Sponge?

It doesn't look like it at this point, but I didn't even know there were gray sponge till you said that so I will watch it. There have been a few of the little white fluff ball sponges springing up in the tank so sponge could be a possibility. I hope I didn't damage the membrane when I picked up the rock.