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Richard2180
Robert5335
It can be anything, including parasites... (when in doubt, I usually soak in Coral Rx or examine under magnification...)
Matthew7977
It is necessary to provide complete information: when the coral was purchased, when the system was launched, what was done with the coral. And also photos.
1) Possible infection - baths with Lugol's solution, very carefully.
2) The coral may have been damaged by fingers during placement in the aquarium - wait.
3) There is something aggressive in front of it, downstream - rearrange.