Hi and welcome to HCM.
It can be anything but probably just a diarrhea
or some anal lesion. Secretion from anal sex looks the same as diarrhea. Only difference is that here can be blood seen after anal sex, but blood can be caused by many anal conditions such as hemorrhoids, fisures, polyps,pylonidal sinus
etc. You can find the true only if you ask him. Wish you good health.