It looks a lot like Hexamita or cotton wool disease... These are common with Angelfish.
Is the fish still taking food or is their behavior strange?
If still eating you can get medicated food... If it's not you might need to either QT it, or dose the whole tank which might be best as the
parasite that causes hexamita is still in tank. It's not too late from the looks of it, but the clock is ticking.
When in doubt I use the med trio that Cory from AQ CO-OP is always recommending. It works just have to follow instructions to the letter.
Thank you so much for your response. I had another vote for cotton wool or columnaris and have already started treatment for that. I look at hexamita just now, hole in the head, per your suggestion as well. And admit it looks like that somewhat too. But the fish is not emaciated or showing any other symptoms they listed. He was however scratching on the heater and darting which both were columnaris symptoms. I am cautiously optimistic that the treatment is working. Today the white patch is barely visible and his energy level has returned and he is eating better. See attached pic. I am treating with aquarium salt and api triple sulfa. I'm at day 4 dose. Being black really makes it difficult to tell if there is an indention or "hole in the head".