I agree with what was just said. That is cyano
. It will go away when water conditions are under control, nitrates and phosphates that is.
Large water changes with ro
water, minimizing feeding with rinsed frozen foods, siphoning out the cyano
during your water changes. It will take time, but it is an easily solved problem.
That then brings up the next part of your issue, your fish had ich! Hopefully you treated in a hospital tank and didn't treat with a copper based med in your display. If you did, don't expect any inverts to ever survive in that tank. But, keeping on topic, fish get sick when they are stressed. This can come from several things, such as living space not being large enough and is common with tangs, but also from nutrient rich water! High nitrates can be stressful and deadly to fish, which is why aiming for under 40 nitrates is best in my opinion.
For a side note, ich is a parasite that has several life stages. You only ever see one, the cyst phase on the fish. You don't see where they are growing in the sandbed or even swimming up from the sandbed to find a new host. To successfully treat ich, this requires 6-8 weeks of running the display tank fallow while treating the fish in a hospital tank during this period with a copper based medication. There are other meds on the market that are all talk, they don't use copper and say they are 'herbal' or 'all natural'. There is no proof of them ever working.