I have superb parameters and I still get this stuff from time to time, looks exactly like yours. Even with phosphates and nitrates below detectable levels while using a UV sterilizer and a goby, hermits, and snails as clean-up crew. There are two culprits in my opinion: one is because my parameters are TOO good for my frags, and therefore I overfeed on purpose. The other is that the tank gets a considerable amount of Indirect sunlight throughout the day (so yeah, I agree your lighting change could be a material contributor).
Also, that type of algae bloom is also associated with relatively new (<1 year) tanks. Hold off on feeding for two or three days and I bet you'll see a significant reduction.
I doubt the bloom has anything to do with your tang or angel deaths. Tangs are algae eaters, they love that stuff, but they are hard to keep alive in new tanks. Angels I'm not as familiar with, but generally they are not good for reef tanks because of their diet. Are you sure you were feeding the Angel food that is appropriate for them? (Yes I realize it looks like you're running more of a FOWLR tank than a reef tank, just curious if appropriate food was being given to the angel.)