It looks like finrot to me, Finfot is caused when the fish become stressed normally by poor water conditions. Do a 20% water change every two days for about a week and it should clear up and dont forget to add stresscoat. Let us know how you get on.
Just make sure you keep up on the water changes to avoid any fin rot problems. Angels love fresh water and it certainly wont hurt any of the other tank mates either. Glad to here the angel is healing.