As @BBawlight suggested, I think they are a type of blood worm/bloodworm (insect larvae or true worms). They most likely got in through the unfiltered hose and/or via the surface of your water.
Here’s some extra information if you’re interested:
Blood worms (Chironomidae), which are midge fly larvae, are different than bloodworms (Glyceridae from the Polychaete class) which are true worms – they bite and are even painful to humans. Blood worms are good swimmers; bloodworms are good burrowers. By your description it is hard to tell which one. Here is a good link of a bloodworm, did they look like that but smaller? Or more like these blood worms? Or were they much deeper red?
If you don’t want those friends back, I’d cover the hot tub with a tight cover that doesn’t allow standing water (you can put a beach ball in the centre of the full hot tub, under the tightened cover to form a tent). Also, make sure to run the tub more often ..keeping it aerated. When cleaning it, get all the water out (like with a wet/dry shop vac). Chlorine alone doesn’t kill them (chlorine hinders the growth of the larvae) – it works best in still water. I believe there are also safe insecticides you can spray. I’d look into getting a hose filter as well.
I hope I didn’t bore you with details.