Just could be a patch of scratched scales...or just irregular scales....not 100% sure but thats what I'd go with.....The angel might have just swam into something and it scratched it scales there
My dwarf gourami had the same thing (what it looks like to me) and I pressumed he had scraped himself when he was in my daughters tank which had gravel. He used to try and stick his head anywhere to try and find some food P.S it has gone now on it's own took about 2 weeks.