I had a Koran Angel that had these. They eventually disappeared but I think a six-line wrasse picked at them and got a 'taste' for his fins and beat him up and he died. They showed up about 2-3 days after purchase and were gone less than a week later. They were described to me as similar to warts.
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication