Assuming it is ich there are only two ways to get rid of it and they both involve removing all fish from the display into a hospital tank to be treated with either hyposalinity or copper. The display mush be fishless for 6-8 weeks for the parasite to go through it's life cycle without a host and die. UV can assist but it is not a fix. It only removes the free swimming parasites that go through the uv.
Once you go through this process and eliminate the ich from the fish and the tank all rew additions will need to go through a qt/treatment process.
Here is a link that gives step by step instructions and details.
This is all assuming that ich is the problem. You want to get a real id of the disease before treating.
Marine Ich - Myths and Facts Copper - Treatment, Use, Problems A Hyposalinity Treatment Process