what is this?

[richm20]

I have these "growing" all over the back of my tank.
I took a picture of one on the overflow.
They are about 1/2 inch long. What are they,are they good/bad, thanks for your help.

 

[Amphiprion]

Simple Scypha spp. sponges (aka "pineapple" sponges).

 

[7faces]

good, bad or indifferent?

 

[Amphiprion]

Meh, indifferent. They basically grow if there are appropriate amounts of bacteria and associated micro-aggregates in the water. They also prefer darker and/or shaded areas. They don't particularly hurt or help much of anything, though they could be considered a natural mechanical filter...