I just recently dealt with leeches. Mine were darker in color, more brownish where this one is whitish. They have sucker parts at both ends, attaching by the rear sucker and waving around or intentionally moving by letting go in front, reattaching the front and letting go in rear - sort of inch worm like movements.
The type I had was definitely parasitic to my fish as I found one latched on to the side of my guppy and a nice lesion left behind where it had fed on it.
I put the fish in QT and treated the whole tank with high salinity then bleach for good measure. They are gross. I got mine from feeding live black worms to a dwarf frog.
Planaria crawl along, they don't do that end to end inching that leeches do.