It’s a demon! One of many of its kind. What it’s “supposed” to do (details fuzzy because I haven’t totally worked out how demons work in this yet) is find humans with an abundance of greed and a lack of motivation and make a deal with them. Pretty standard, “all the riches/fame/power/whatever for the rest of your life, and your soul in exchange. But we’ll only come to collect after you die, and don’t worry, we promise you won’t need it anyway”.
And then the worm pulls some strings, alters the course of events (not always in a desirable way. Watch your wording when making devil deals, friends), and also quietly arranges for the deal-maker to die. Preferably in an ironic manner, though that doesn’t affect the contract at all. It’s just for the worm’s own amusement.
The worm Morgan has was baited into making a deal with her, and wasn’t quite quick enough to spot that she had set up an elaborate binding ritual on both “sides” of the dimensional divide (our world and demon world. Mages can enter that world as a shade, though not easily). She basically trapped it and, through somewhat unpleasant means, forced it into a contract on her terms.
It is now pretty much fused with her until she dies, and can’t do a thing about it unless it somehow gets outside help (not easy, because Morgan accounted for this in the contract). It’s basically a leech with practically infinite capacity, constantly drawing a small amount of her blood into a pocket of the demon dimension and preserving it indefinitely.
Tah-dah, Morgan is now practically impervious to bleeding to death and has a constant companion to talk to! A companion that fucking hates her, but there’s nothing it can do to stop her from taking selfies with it and telling it stories it’s heard 500 times already.
Oh also, the little blood-form thing floating around Morgan isn’t the greed worm itself. It’s its equivalent of a shade in our world. Basically just a projection that can interact in a very limited way. On the other side, Greed Worm is pretty huge, corporeal, and looks something like this:
(Old art and the design has changed, but this is pretty close)