Angelfish has Diarrhea?

what are you feeding him? what's your tank temp?

Is he on a vegy diet?

If the angel is behaving normally, I would not be overly concerned. I do, however, suggest checking your water parameters, doing a series of small water changes, and setting the tank temp to 80 for a few days if it is set lower than that. Skip feeding for a day to give the angel's system time to clean out and all should be fine. If not, it may have a parasitic worm. These can be a problem. They can arrive with new and unquarantined livestock, on plants or in live food. If you have that concern, you will to treat the whole tank since, absent a specific lab work up, you do not know if this may have spread to other fish, be luring in the shadows.

Charlie