White creature in new tank

I just set up this tank yesterday with plants from existing aquariums, I woke up this morning and found it swimming around. Is it bad or should I not care?

Probably a planarian (a kind of flatworm). Not particularly troublesome as long as their population doesn't get out of control (easy to avoid by keeping the tank clean and not overfeeding). Fish will happily pick them off. They can potentially be a nuisance if you keep shrimp, but other than that I wouldn't worry. I'm pretty sure most established tanks have a population of these critters (and a bunch of others, too .

Never mind, just saw the video you posted in another thread. Definitely not a planarian.

What would it be?

I honestly don't know, but it could be a damselfly nymph, like others have pointed out. Or some other type of larvae. Does it have any discerning features (legs, spikes, wings, anything)? You might want to get it out before you add any fish or shrimp, as nymphs are predators and can take down small fish with ease. Relocate it somewhere if you can, as they're very useful animals in the wild (they eat all kinds of harmful insects like mosquitoes, etc.).

Unless of course you want to keep it and watch it grow into the adult form of whatever it is ?