Those in your greenhouse are a good thing. I kept them as pets for a long time, their appetite is gianormous. That lil guy can take care of any bug problems before they start so yeah, I'd say good omen. Plus they make really AWESOME chirping noises at night. Surprisingly enough, it's either a European Tree Frog or an American Tree Frog, scientific name, depending on where you are.
There isn't much of a difference in the 2 species. I get Gray Tree Frogs in my peach trees and you can hear them sing at night.