It could be stress, but more than likely it's an indication of parasites and/or worms, which are very common in beardies, and can be carried by any type of live feeder insects, or even on fresh veggies and fruits. The fact that his poop is very runny combined with the red colored urates (could be a bit of blood) is an indicator of parasites. If he's having any other symptoms like lethargy, sleeping more than usual, lack of appetite, vomiting, etc. them he definitely has something going on and needs to see a reptile vet.
He hasn't had any accidents or falls has he? Any time there is an indication of blood in the poop you also have to worry about an internal injury if he has had a bad fall or accident. If not, then it's most likely parasites. I would call his reptile vet and tell them what's going on, and see what their procedure is for dropping off a fecal sample for testing, then bag up his next bowel movement and you'll most likely have to drive it to the vet pretty quickly and just drop it off. Usually they'll test it without you bringing him in, and if he tests positive for anything then they'll have you bring him in to be examined and put on the appropriate medications. It's definitely worth having a fecal done since parasites are so common and it's pretty inexpensive to have done, as are the medications. It doesn't necessarily have to be a "bad" batch of feeder insects, it can just be insects that picked up a parasite or bacteria during shipping, or more commonly at the pet shop, or it came from any veggies or fruits he's eaten, people actually get very sick from raw veggies and fruits all the time because they don't wash them well enough.
