HLLE is correctible, but not without some effort. Generally, the longer you let it go, the harder it is for the fish to recover. Once it sets in pretty good, there will be some scarring.
The good thing about HLLE is that it acts slowly, so you have some time. The bad thing is that this means it takes a while to see if you've made any progress.
YES, Pomacanthus Angels REQUIRE sponge. If you do not provide it, they will eventually get sick and die. Ocean Nutrition Angel Formula works well for this and it's dirt cheap. It also has lots of good stuff that the other fish will benefit from.
Hippos and Emperors are both omnivores. The hippo, unlike other tangs, requires some meat in his diet, although the lion's share of his food should be plant matter. Be sure you are occasionally feeding meaty foods, such as Mysis shrimp, chopped Krill, etc.
For both of these fish, you can also throw veggies from the kitchen into the tank for variety, provided the veggies are washed and don't have pesticides all over them. I'll steal hunks of broccoli, cucumber, etc from my fiance before she cooks them, stick them on a seaweed clip and let them graze.
Finally, supplement your food with a vitamin A supplement such as Zoe or Selcon. I use both additives and alternate. Every time though I also pour in some Garlic Guard. Soak your meaty foods as well as your seaweed in these additives prior to feeding.
Check those nitrates. Above 15... do water changes.