Mucus is normally present in the intestinal tract and, when you see it, it looks almost gelatinous and there might be blood present as well. This indicates gastrointestinal upset.
You need to see the vet to rule out parasites. Many people don't and they go by site alone. Even if you have already wormed her, this is a concern because all wormers don't work for all types of worms. Giardia and coccidia, two protozoan concerns, are also something to worry about.
Parvo is always a concern if he has not been fully vaccinated so be sure and see your vet quickly if you see bloody diarrhea, vomiting or lethargy.
Let me know if you have more questions about this.
I hope this helps!