Not sure what everyone here is looking at but those are not tunicates. They are some type of anemone--majano or similar. Second guess would be a colonial hydroid--the type that tends to resemble zoas, not the digitate kind.
Both options are invasive pests so that would explain the rapid spread. Note the central mouth and the small tentacles around the perimeter of each radially symmetrical polyp. Tunicates have a central "mouth" and one side opening for water to flow through as they filter feed.
Not all biodiversity is good! May not harm anything but can potentially cover all your bare rock eventually. No advice really on how to get rid of them at this point. You could try smothering all you find w/ kalk paste and see if that works.
Yes, they are tunicates. They may look like tentacles but are not. Looks can be deceiving.If these were a coral, anemone, or hydroid, they’d have a uniform number of ‘tentacles’ (they’re not tentacles, they’re individual zooids or animals that are sharing a central tunic/oral siphon - the chamber that takes in water). If you look at the photos, they do not all have the same number of ‘appendages’. The other problem with the assertion that they’re a majano is that there are several that are very clearly on the underside of the rock and receiving no light. I think I may have gotten the genus wrong in my previous post, but they are very clearly tunicates. Look at the photo here, it’s very clearly either this species, or a closely related species:
Botryllus schlosseri Star ascidian, golden star tunicate
Botryllus schlosseriis commonly referred to as Star ascidian, golden star tunicate. Difficulty in the aquarium: There are no reports available yet that this animal has already been kept in captivity successfully. A aquarium size of at least 100 Liter is recommended. Toxicity: Toxic hazard unknown.www.reeflex.net
https://www.underwaterkwaj.com/uw-misc/tunicate/Tunicate-sp1-jj-6mm-6232-070416.jpg
At first blush, I thought they might be some sort of Manjano as well. The zoom function on any browser is very helpful.
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