Unidentified posible plage?

[Daan]

Hi people, does anyone have any idear what this is? And if it's bad or not? It is the white stuf growing on top of marine blocks down in my sump, where it is dark. However I also have seen it in the main tank, also on the side of corals. Thanks in advance!

 

[ying yang]

Looks like sponges,they like to grow in dark / shaded areas

 

[Algae_Farmer]

Concur with @ying yang, looks like sponges. Its hard (for me anyway) to tell what type from the pics. Good or bad is complicated, depends a lot on your other livestock - probably great for free biofiltration in a FOWLR tank, but in a reef tank they can compete with corals for resources, or even leach competitive toxins that inhibit corals. I get a lot of pineapple sponges in my sump, which are fine except they consume nutrients. Yours don't look like pineapples though...