Last year I bought a plastic great blue heron and it worked

At the end of last summer a green heron took up residence. Do I buy some kind of predator decoy?

I see they sell owl, fox, coyote, snakes, and other decoys. What do I need to do?

I have used a plastic blue heron decoy for the past 4 years and it has worked for me keeping all herons out of my pond. I am not sure if it will scare away a green heron, but since the blue is much larger it is worth a try.
Here is an article on the subject. Good luck.

http://www.okanagankoi.com/Combating-predatory-pond-birds-a-18.html