Is this a worm on my Mystery snail???

[BettaFishGirl]

I just got 2 ivory mystery snails and noticed this weird worm thing... is it harmful??? Sorry for not the best camera quality. Its kind of flat and was moving all over the snail. Please answer soon.

 

[BettaFishGirl]

It's gone now for some reason, and I'm thinking it's planaria??