It's plain green peas, like you get frozen or in a can, with the shell removed. This helps with constipation issues. Epsom salt also helps with constipation, 1 teaspoon per 10 gallons of tank water, increase by a teaspoon per 10 gallons for a couple of days for a maximum of 3 teaspoons per 10 gallons.
Usually when angels act that way, it's an internal protizoan. Loss of appetite, tending to hide towards the bottom or back corners, and white feces are major signs. I have a 3 year old silver super veil that is going to get medicated starting tonight, been showing symptoms for a couple of days.
Here is what I do, I've done it before & saved at least 10 angels this way, along with an oscar. Put the angel in a quar tank, I usually use a 10 gallon. Increase the temp over a day or so to 90-94F. Treat daily with 40 mg/gallon metronidazole, with 50% water changes daily. If the fish isn't eating, don't feed for the first 3 days. After 3 days, get some frozen brine shrimp. Take a portion about the size of a few match heads, and sprinkle on some metro. You have to eyeball this one, make the shrimp look kind of like a tiny powdered donut. Once it thaws, mix it in & feed. Sometimes they still don't eat for a few days, but once they start to mouth the food & spit it out, the meds seem to get in their system real good. Continue medicating the water & food for 7-10 days.
Metronidazole is sold as flagyl, het-a-mit, and a few other names I can't recall offhand, used to treat hexamita & hole in the head. They usually suggest 20mg/gallon, this often isn't enough. Metro deteriorates in 8 hours, so you could do twice daily water changes. I've done this on the weekends when I have time.
I've also added epsom salt while using metro, this helps with any constipation issues, cleans out the digestive tract. I start with 1 teaspoon per 10 gallons of water, increasing by 1 teaspoon daily for 2 more days to bring it up to 3 teaspoons per gallon. You will have to adjust how much you use when doing water changes. You may want to give this a try.