Thats really weird... do you have an algae problem already? I had a filter that used to cause bubbles exactly like that. They only turned green when I had an algae bloom. (first tank had too much phosphate) but I have never seen anything that shade of green. Maybe you have a metal that, when oxidized in water, causes it to have a slight green sheen thats only visible in bubble form. Just throwing out ideas not really seen something like this before... either way as long as they are not harmful I wouldn't worry too much.