Nice looking tank.
If the Betta's not eating them, then I doubt they're daphnia. Daphnia make Bettas go nuts, in my experience. They must be stuck on the glass? If that's the case then they might be planaria. How much and how often do you feed the Betta? If you're not target feeding him, as in putting it right in his face, and only giving him an amount of food equaling the size of his eye (which is close to the size of his stomach), I would try that, it'll cancel out any offerings for scavengers and their numbers will plummet before they completely disappear.
It's also entirely possible they're coming from your water source, make sure it's well filtered or chlorinated before being treated and added to the tank. Using natural water sources like pond or rain water that has been sitting out for a little while before collection can bring in all kinds of interesting critters. I actually leave containers of dirt out in the rain then collect the "bugs" for live fish food.