tank is not really over stocked
the physical condition of your angel is telling you something is amiss ...
The easiest - & likely most effective - intervention is to increase water changes.
Even with 50% weekly water changes it is easy for nitrates etc to accumulate in a tank & gradually affect fish wellbeing.
I can't tell from your photos how large your angel fish are, but at ~2 years they should be full grown with at least 4-5inch bodies (not including fins) - note the discussion at the end of this
article (it's short
😉 )
A second easy action is to adjust diet - if you can't provide frozen foods, then look at various cichlid pellet type foods available in your area
These often store better than flake, not just in your home but also along the distribution chain, if possible buy foods with manufacture dates, if there is a visible "lot number" or "production number" you can contact the manufacturer for dating information, store fish foods in fridge or freezer, especially once opened.
Offer a variety of foods, also try to source foods from various manufacturers.
Can This spread to other angels?
please take some time to read the linked article - it provides some very good information.
In short, yes!
BUT this sort of disease is not highly infectious in the way of ich or other external parasites or bacterial conditions such as columnaris etc
HITH or HLLE is a longterm "disease" (think of it as "chronic fatigue syndrome" if that helps) - it's observed when conditions are not quite right for a long time, rather than just a few weeks.
Anecdotes abound ... one fish lab (I don't recall what sort of research) had HITH/HLLE show up in their tanks after they'd installed a new water system - they'd always run the tanks with "purified" water in order to better control tank parameters - this was not a case of "poor" water parameters but rather "something missing" ... the situation resolved when they returned to their previous water system (not the first solution attempted
😉 ).
That anecdote aside, most hobbyist tanks fall on the side of lesser water quality
🙂