There are a few possibilities for what is going on. Superworms are high in chitin, which dragons can't digest well. They're fine for some other lizards, but not beardies. The chitin accumulates in the intestines, and eventually causes constipation and appetite loss. Sand as a substrate is another potential cause of this. Dragons often accidentally ingest some as they are eating or exploring. It, too, accumulates. I'll give you a measure to try for this.
Buy some baby food prunes and some Pedialyte (yes, the kind for human infants). Drop a dollop of the prunes on the end of his snout. Most of the time, reflexes will cause them to lick it off. Wait about 45 minutes, then prepare a shallow warm bath consisting of 1/2 warm water and 1/2 Pedialyte. Soak your dragon for about 20 to 30 minutes. After the first 10 minutes, with him still in the water, gently massage her underside from front to vent for an additional 10 minutes. That may be enough to help him pass anything in his intestines. Be sure to supervise closely.
The smelly, runny poop often indicates parasites or a bacterial infection. Normal is neither runny nor hard and dry. For that reason, it would be a good idea to take a sample of fresh poop in a clean plastic bag to a reptile vet for a fecal exam. This link will take you to a directory of reptile vets:
If the soaks and massage don't help, Mongo may be sick. In that case, he will need to be seen by a vet. Regardless of what happens, I'd stop the superworms. stick to crickets, Phoenix worms, or silk worms. Here is an excellent site that will show you all the variety you can feed. I refer to it often.
I would also switch out the sand for reptile carpet or ceramic or slate tiles. Despite the fact that pet stores recommend them, the sand and other loose substrates almost always lead to trouble in the long run.
I hope it will turn out that Mongo is just a little constipated and the above measures will help. If you have more questions, just let me know.
My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back, I am happy to address follow-up questions. Please remember to rate my service after you have all the information you need. I will greatly appreciate a positive rating as that is the only way I am compensated. Thank you!