"A little green" is a bit vague. There should be 0 ammonia in the water when you test. 0. The only exception I can think of off hand is the .25 reading you use Prime as your decholorinator. If there's ammonia, it sounds like your tank is not cycled.
I would recommend stepping up water changes until your ammonia consistently reads 0 (or .25 if you're using Prime).
When adding a conspecific cichlid, it's probably a good idea to take them all out of the tank, rearrange it, and then add all of them. That should help settle territory disputes. This can't be a parasite, if you've added nothing new to the tank to introduce one. That leaves stress and illness. Since you've added a fish to the tank, they can all be stressing each other out. How big is the tank in question?
I keep my nets in air, typically. That said, I only have 2 tanks (almost talked my wife into letting me set up a second 10, third if you count the QT, with a betta and CPDs) running right now, and don't worry about cross contamination, because new fish typically go into a quarantine tank, and are pulled out after a month or so, if healthy. Granted, my last three species' acquisitions, and likely my next (EBR/rummynose tetras/koi angelfish and otos, respectively) didn't/won't go to QT, but I know it's a calculated risk.