I thought this was funny, it's an M. digitata colony that's growing a tad faster than a sponge inhabiting the same rock. Any ideas for a safe biological control would be appreciated.
I am no expert on sponges, but I do have one that is a black/brown that grows in strands like the one in your photo (yours is much prettier than mine) and I just snip it off and take it out of the tank when it gets out of hand. Mine is not growing up in a stand of coral though, no ideas on how it will impact the coral with toxins and such. Everyone in your photo looks well why not just leave it like it is until it seems it is doing damage?
What's the problem? Eventually the digitata (or other corals) are going to completely shade the base and it'll die off anyways! let the sponge live!!!
One option you may want to try is just pull the sucker out of water and let the sponge dry out, from what I understand corals are much more resilient to that sort of abuse that sponges... do what Rich says and yank it off. Or get some Anglefish that eats sponges, if you have enough of it they might even leave your corals be!