Zoas and paly can morph under different lights and or different water conditions. For example blue hornets are most likely nothing more than purple hornets that faded out under intense light. Most likely you're lighting/water conditions were different than those of the person who sold them too you, and as a result they morph.
FYI, I've never heard of Green Lanterns, nor are they listed on ZoaID. I would say that what you have are green button polyps. JMO