Are these hydras?

If so, any suggestions on proven treatments, (plant and assassin snail safe)

Yes I believe they are hydra.
Add aquarium salt, 1 tablespoon per 10 litres. Ensure it is dissolved before adding. Leave the salt in there for two weeks. If you do a water change you will need to replace the salt that is removed (for example, if you change 50 litres of water, also replace 5 tablespoons of salt).
After two weeks remove the salt graduallt. Do a 10% water change daily fir a week, then a 20% water change daily for a week, then return to your regular schedule.

They ar ehydra that have been feeding on algae and turned green. Normally they are clear, cream or pale brown in colour.

As mentioned by Naughts, salt should do the job but you normally only need it in there for a week.

Try 1 heaped tablespoon of rock salt for every 20 litres of tank water. If the hydra don't die within 24 hours, add another heaped tablespoon for every 20 litres so you have 2 heaped tablespoons per 20 litres.

I had hydra and they spontaneously disappeared - I know I manually removed some through siphoning, but they spread like, well, hydra, so it must have been the aquarium salt I was adding before I realised I shouldn't be using it with frogs!