Join Date: Mar 2018
Location: Tamer Valley, Devon, UK
It's a Chlorohydra or Hydra viridissima, which is pretty harmless; the green coloration is from a symbiotic relationship with algae which differentiates them from the usual suspects. It is on the glass near the surface so as to get light for the algae. As far as I'm aware, Hydras, are only dangerous to fry and baby shrimp and even then not exactly a horror as they mostly feed on plankton.
Some fishes eat them. But if you don't get rid of them do keep them in mind if you ever give someone some plant cuttings!
As someone else said, do not squash them, if you remove them physically be really careful as they do grow from pieces of themselves in the same way that planaria do.
I've never found any kinds of Hydra in my tanks, I'm assuming fish have eaten them if they've ever gotten in.