Hello Friend, I don't think so that it is a worm or a slug. This more looks like a larvae of an insect. I am pretty sure that is is Black Soldier Fly Larvae.
If you search up "Black Soldier Fly Larvae" you would see lots of information about them. Or you can just search up "Black larvae" and see which image matches to yours the best.
Hope this Helps!!!
~Your Friendly Brainly user
~RecklessChicken
