Yes, not sure if it's tape worm or round worm, you'll need to get de- wormer medication. Meanwhile do regular water changes and vacuum the gravel to remove worms and their eggs.
Sorry, not a worm, just a very long platy poop. Platies poop like this if they eat a lot, so since you just got him, he probably overindulged at the store. So give it a few days. If it stays the same, try a dewormer.
But there is a reason platies have high bioloads. Seeing the monster poops on them, now you know why