This is on some lava rock type stuff in a FO tank. You can see a Heliopora or "Blue coral" skeleton in the background, and the patches of sponge on the brown rock. About the same color. This sponge turns yellow if left in the dark. No live substrate was ever added to this tank, so I presume it came in with the water from a fish bag. There are also amphipods galore in the sump of this tank.