What is this black gunk on my LR?

[JJFly]

This is what we use to forum for, RIGHT? HELP! Please... I don't know what this stuff is. My tank (90gal) has been up and running 2 weeks now. It is still cycling, no fish or inverts... This black gunk looked like a form of macro algae at one time. I've been trying to scrub it off the rock, to no avail. What am I to do with it. Leave it grow or remove the rock and scrub it off? Other suggestions?

 

[XXFirefighter]

I would leave it and see what happens...

 

[ohio reefer]

i think it might be dying sponge. if it stinks really bad thats what it is.

 

[daclozer]

it looks like an encrusting type of sponge to me. I have a large piece of rock that started with a small patch about the size of a silver dolllar, it is now about as big as an apple. it is flat, black and grows slowly.. no harm, it is actually very attractive to me and it is a filter type feeder..