What does that plastic watering can look like? A photo might help us come up with a way to clean it.
That green stuff is algae, I wash my waterers in a bleach solution and rinse well to get rid of it. Don't let your chickens drink that bleach solution, it's dangerous, but as long as you rinse the stuff you wash well it won't hurt them.
If you can't get to it with a long handled brush we may need to do some thinking. So what do you have to work with?