Yellow rock encrusting sponge
They are generally called pineapple sponges for obvious reasons. They are common. They vary from white to yellow and even orange. In rare cases they can get out of control but are easy to kill if necessary. They are usually a good sign that your reef is doing well. I believe they are filter feeders and not photosynthetic.
If it's a true encrusting yellow sponge, then that's a really good thing if you like sponges.
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