hmmm generally that is a sign of decline in the sponge... If you knew 100% that it was nonphotosynthetic I would trim off the piece of the sponge and place it in the shade... but most sponges need a little light. So here is what I would do, get some sharp scissors and cut away the algae "infected" areas (throw them out). Place the sponge in an area of good flow, but not to powerful or the sponge could be damaged by the pressure of the water, so maybe off to the side of a powerhead, instead of directly in its path. Though often times if a sponge starts to decline there isnt much you can do, if it really starts to die, just cut of most of the sponge down to the base and let it recover naturally (in theory it should though slowly).
