I gave slippery elm to ChiChi when we suspected she may have had ulcers from long-term Doxycycline use and her autoimmune issue, and it helped some with her appetite and willingness to eat for a while. I don't think it masks anything but rather is soothing for things like that and may aid in healing. If I recall correctly, it is safe to use over extended periods of time. For diarrhea like Max has from possibly scavenge-eating, the fiber it provides can help with either diarrhea or constipation, and for those kinds of issues is usually given until the troublesome symptoms resolve. Because it coats the tissue, it can block some absorption of medications, so it's best to give it at a different time than medications or other supplements.
The mucousy stuff with the poo may indicate that things in the GI tract are still inflamed or irritated, so I'd keep giving it for a few more days and then try without it again to see how he is. I also agree with Connie that the probiotics should help too.
I tried the slippery elm for Dixie and Cinnabon when their drug-induced kidney damage symptoms got bad at the end of their lives, but in their cases it didn't seem to have as much positive effect. Nothing much did though.
Hope that helps, and hope Max has perfect poo again soon.