Those yellow sponges as an indicator of overall tank health in my tank. If your iodine and Mg levels get low, it stops growing and the yellow fades. These yellow sponges are also known to grow quickly attaining goIf ball size and branching out smothering some nearby coral. If this happens, there is good success controlling these sponges by injecting them with air using a hypodermic needle. Vinegar works well too via injection.