Yellow Slime on GSP, Sponge?

[Mflores888]

I was moving around some corals and rocks and noticed this was growing all over the rock where my GSP is. It also started to grow on the back of my tank wall. I'm guessing it might be a sponge of some kind but I haven't seen a sponge go on glass before.

 

[SHNICI]

Yep, sponge.
Have on mind exposing this one for too long on air, you will kill it (the sponge) after it will start to rot and smell very bad, which tend to kill the GSP.
if you already put it in a tank, observe the sponge, if it change the color, I advice you to peel it off, to be able to save the GSP.
But if nothing change and the GSP isn't bothered, leave it as it is, if you do like it.

 

[SHNICI]

About the sponge, probably your silicates are a bit high, that encourage the sponges to grow faster, usually they are filter feeders, but few of them aren't reef safe and tend to kill corals (Be careful if you get black sponges, they are mainly problematic).
Most of them ( yellow, black and white ball like) can destroy the LR from the inside consuming the calc and the chemicals from, after that the LR is coming brittle and unstable.