Looks like a cirolanid! Find them and flush them. They attach to and feed off your fish.
Read this
article for more info. Excerpts from the article.
"The same species will fasten on to larger fish and eat its way into a major blood vessel where it will remain for some time sucking blood and eating tissue. When sated it will detach from the host and swim away.
Tropical species not infrequently show up in reef tanks either riding on a fish or in a piece of live rock. Often the first the aquarist knows of them is when they see the isopod on a fish. Murphy's law is active here; the bug will never be on a cheap or expendable fish. The problem is how to remove the isopod from the aquarium. If the bug stays on the fish, the fish needs to be captured. . .
If YOU can catch the fish, the isopod may be removed with a pair of forceps. Carefully!!! Upon removal the fish should be isolated in hospital tank, and treated with antibiotics until the wound heals. The bug may be disposed of. Carefully!!! I had a student who was holding a three centimeter cirolanid that we had just collected in her clenched hand. The bug cut through the flesh of her palm, dug in, and started to eat HER. Her response was rather impressive. So was the tenacity of the isopod, it was HARD to remove it.
If you notice a cirolanid in your tank, and it leaves the fish, there is almost nothing that may be done to catch it. They are very fast, small, and quite capable of avoiding a net. And if it is a pregnant female (remember all isopods have brood pouches), and the brood hatches, you have REAL problems. There are only three courses of action in this situation; and I truly am not jesting about these responses. The first is to remove all the fish from the tank and wait the two or three months until you are certain that all the isopods have died from starvation. The second solution is to effectively nuke the tank. Remove all live rock and discard it as the isopods may hide in it, and as some of the isopods bury in the sand, you should also remove and discard the sand." End of excerpt.
Other folks on this forum have found them shortly after receiving some very fresh
LR. They have been, to their knowledge, successful at catching and removing them from their tanks. It takes time and patience.
If you search on the term "cirolanid" you should find some posts about catching these parasites. Good luck!