FWIW, I doubt one dead fish would result in a tank crash assuming that it is adequately sized for a dwarf angel and is fully cycled. Your CUC will likely consume the carcass before any real damage is done with only a slight blip in your parameters. IME, dwarf angels are fairly social fish so long as nothing is harassing them and even if hiding in the rock work, they will usually reappear due to their voracious appetite.
Has he been missing for a few days or is he showing himself occaissionally?
How long have you had him?
What other fish are in the tank and does he seem to have nipped fins?
What size tank and how long has it been running?
Has he shown any signs of disease such as discoloration, spots/bruising, loss of appetite, etc?