Black, slimey, smooth

Hi i was wandering if any one has an idea what is growing in my tank, on one rock where there is some brown mushrooms, there is black stuff growing, one part of it looks like a mushroom cause it is round and has a central hole like a mouth, but another patch of it is totally irregular shape like a splodge and has 2 or 3 little holes in it, to touch it is smooth and slimey, im going to try and upload pic, first time of doing this so hope it works, my photography skills needs work

 

Black encrusting sponge.

In some people's tanks, the conditions are right and they spread rapidly. In other people's tanks, it will stay the same size forever or even slowly die off

 

is it a good thing though????????????,,,,,

also havent got a photo but i have like a white fury clump formed and there is tiny particals of rock and sand all stuck to it, its like a cocoon????????? any idea

 

do 2 antennae like things occasionally stick out of it? If so, it is a harmless tubeworm

 

above that black sponge(IMO) and to the left, there is an aiptasia. I would get rid of that sponge and the aiptasia. Someone told me a long time ago that black sponges were bad and they take stuff over. don't know how true that was but, hey, I'm just passing it on!