Myxazin is Formaldehyde based. If you dose it in the presence of corals, shrimp, crabs or snails in a marine system, they are gone, finished, dead! Along with a lot of micro and macro fauna and flora on the live rock. As Truck said
There is no medication that is proven to work, that is safe to add to the main tank as almost all are Malachite Green, Methaline Blue, Copper, Formaldehyde or Potassium Permanganate based. All of these will kill invertebrates and corals
You need to either treat in Quarantine, or sit back and watch the diseases progress, hoping it does not kill too much
Some recommend treating in a bucket with fresh salt mixed water in, changing the bucket over to a new one daily, and sterilising the equipment in between. When I tried this though, I lost all my fish. while I cannot prove it, i suspect it was due to Ammonia spikes
All the best
rabbut