Quite a bit of sponge growth in our tank is unplanned, coming in as a hitchhiker on the rockwork. If you have some, take some time to enjoy its design, its texture. Remember that sponge will not survive out of water, so if you remove the rock from your tank for any time at all, it will die and can cause an ammonia spike in your system. You may even have to scrape it off, but your better choice is to keep the rock submerged at all times, even in a separate container while you arrange your reef to suit your design. This one is Reef-safe-ish; it does grow at an aggressive rate and can outgrow neighboring corals to their detriment.