There are hundreds of types of tiny little worms, and very few of them are harmful. If they suddenly seem to appear in your aquarium they're probably not parasitic, so your axolotl will be fine.
Most of the tiny little worms in aquariums eat detritus - organic stuff that has sunk to the bottom. For example, axolotl poop, left-over bits of food, dust that has landed on the water surface, etc.
Basically, they eat junk and that's very nice of them because you don't want too much junk in the aquarium.
If you suddenly see a lot of them, that might be of some concern. Not because of the worms themselves (since they're harmless), but because an increase in worms only happens when you have an increase in junk. So then you would have to think "where did all the junk come from?" and maybe take action.
A few tiny little worms here and there are no problem, and they keep your substrate nice and fresh.
