Unexpected worm guests

[GraceTheNoob]

Not sure if anyone knows what these little guys are? They look like tiny white/translucent worms wriggling around in the water.

My tank does not have any fish in because I'm still cycling. There's at least 3 of them but I'm sure there's probably more (I've only ever seen 3 at a time in the same field of vision but it's hard to keep track of them with the filter and the bubbles). They don't just live by the filter, btw, it's just where they're most visible because of the colour contrast.
I never introduced them myself, so I'm wondering if they hitched a ride on my plants, substrate, or gravel? Everything, including the tank itself, is brand new.

Part of me thinks they're kind of cute and interesting to watch wriggle around. The other part is worried they might be harmful to my plants, and eventually my fish?
I figured when I do eventually get fish in there that the fish will probably eat them, but I don't know if they'd give my fish a disease or not!
I noticed when I was cleaning my gravel today that I disturbed one, I don't know if that info helps or not.

Thanks in advance!

 

[Aiken Drum]

Looks like some kind of detritus worm. Maybe planaria if it has a triangular head.

It would be easier for you to look up photos and identify them than for us to to identify them from the photos. Google detritus worms and planaria and see either look like what you have in there.