Usually mucousy poop is caused by irritated intestines. Has this pup had normal poops yet? Is the pup normal in every other way? How's he eating? It may be from the deworming, that can be pretty hard on their systems. If you've got some plain yogurt, you might want to mix it into their formula/kibble mash, it certainly won't hurt and can help replace some of the good bacteria that's been stripped by all the diarrhea they had.