Welcome to Reef2Reef!what biota is this? type of pest or not? reef safe or not? Thanks
Those are colonial tunicates - some species can be incredibly invasive (to the point of growing over live corals), but others are totally harmless.
If possible, I would isolate them so they don't spread too much, but you should be able to just scrape them off/remove them if you don't want them.
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