Sounds a parasite for me also. I don't recommend live or frozen foods, I know that the fish love them, and a lot of members here use them everyday with no problems, but IME with Angels, frozen blood worms or live food make them constipated some times or the live food canbe infected with a parasite...
I lost some Angels in the past, kind of like you said, white or grey long string poo, they stopped eating, and from there was hard to save them.
If they are still eating, you can traet them with medicated food for parasites, you should be able to locate some at your local fish store, or you can buy some online from Angelplus... If they are not eating, sometimes using ParaGuard (I think that is Tetra or Jungle) to treat the whole tank helped me sometimes.
I only fed my fish now with Life Spectera pellets and they have been really healthy... some time I give them dry flakes from Angel Plus to mix a little, but no more blood worms or live food.
Lats thing, try to do water changes of more than 10%, like 30 to 40% weekly, your fish will be more happy.