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If you poke at it does it squish or it it soft? If so I would say sponge. If its hard and doesnt come off easily it could be coralline algae. Is this a new rock or anything?
Scypha or commonly referred to as pineapple sponges volunteer often on live rock. They are harmless, unless it gives you the certainty you can now grow sponges in your tank. These little guys grow under conditions other sponges would find lethal.
Thats not what I meant. You have to have pretty good water to get these to grow.
Repost;
The calcareous sponges, use calcium to form their "skeleton." (spicules) The so called "pineapple sponges" are Class Calcarea sponges (as opposed to Class Demospongiae, which use silica and is a far more diverse group.) But whether they are from the Genus Scypha (the snap id of pineapple sponge) is a difficult question. I suspect these sponges are showing up more frequently, due to the widespread use of salt mixes with higher than normal seawater levels of calcium.