ectoparasite

Why is there a coral growing on my fish? I’ve never seen a parasite like this before. On close inspection, it looks almost exactly like a coral polyp. The fish is active, and somehow managing to eat well. Ph: 5.5 Kh: 8+ Nh3Nh4: 0 No3: 5 No2: 0.3 Has anyone ever seen this before?