I'll keep an eye.Could be the start of a fungal infection then. Fungus isn't fatal so I'd just keep an eye on the angel for now to see if it goes away on its own or gets worse.
They eat well, water could be better. I test that tank about as much as Frank his.Also lympho is a possibility. It is usually on the fins, but have seen it elsewhere on fish. It is not treatable and usually goes away with a good diet and clean water.
Newest residents were rescues 8 months ago or so. They were observed for about a month before going in. No issues noticed.I suspected fungus over lympho because the angel has been in the tank for 3 years, but it's still a possibility.