Gray on live rock

[Lcombs]

I have this gray stuff on my live rock. It doesn’t seem to be spreading but I don’t know what it is, I’m new to the hobby and have tried searching online but not coming up with anything similar. Any help would be appreciated

 

[Lcombs]

It’s completely smooth, no ridges or holes in it.

 

[Murica]

It looks like a sponge or bacteria build up, harmless either way.

 

[NB Reefer]

Looks like a sponge. #reefsquad

 

[saltyhog]

Agree with sponge, I have the exact same one in my tank. That one seems to like light more than other sponges. My other sponges tend to be tucked away, out of the light more. This one however is on top of a rock fairly high in the tank.