This is bizarre: when the lights came on, the hydra were simply...gone. The greenish smears on the glass are nowhere to be seen. Would they migrate en masse or did something have a feast? My money's on the mystery snail if so.
For future reference, do they attach themselves only to hardscape or will they anchor on plants too? I assume sand isn't a good grip.
They'll anchor to anything they can, as far as I know... including giant shrimp and pre-filter sponges.
And they move, so keep that in mind. Just because you don't see them on the glass doesn't mean they're not still in the tank.
I've used this - https://www.amazon.com/Merck-Safegua...4-3e7a9c0027d0
Others have suggested against Fenbendazole, I advocate for it in a shrimp tank. Used it a few times to take care of planaria and hydra in my tanks. I'm very bad about quarantining plants...
The thing with Fenbendazole is it will kill some of your snails, but Malaysian trumpet snails, pond and bladder snails will be fine, your chopstick might be fine as well but you'd need to research. Not sure on the Mystery, you'd have to check that as well. Nerites would have to come out, and you'd want to run carbon for a while before trying to introduce them back. And only add one back at a time to make sure the tank is safe, I didn't do that and lost a couple nerites in a 10 gallon shrimp tank early last year. I recommend medication in this situation because manual removal is tedious and not guaranteed to eliminate the problem. If they are in your filter, hiding in planted areas where you may not see them, etc.., you're going to keep having Hydra. Hydras reproduce asexually - the mature hydra will form a "bud" that will turn into another hydra and then break off. Imagine a cell splitting, basically. Also, if you smash them and break them apart, each smashed piece can regenerate and become a new hydra.
Just providing the flip side of the coin here so you can weigh your options.