Definitely some form of chewed up/wet pellets. As already mentioned, due to a hamster’s anatomy, they are incapable of vomiting. Most small rodents are as well. The muscles required to return the stomachal contents back up, cannot he contracted in rodents. Here’s a good explanation on why rats can’t vomit (it applies to hams as well)
Why rats can't vomit
It’s nothing I would worry yourself over. It looks like the pellet matter is the filler blocks in food, not well liked by hamsters! Best wishes