More than likely algae. Algae live off nutrients. If you overfeed, you can increase the chances of having lots of algae. Try reducing your feeling for a while and see what happens. Also, leave the lights on from no more than six hours a day. If your tank is anywhere near a window, put some backing on that side of the tank to reduce the amount of light the enters the tank. As for Oscars behaving oddly, this is something they do. I couldn't possibly answer your question reliably as to why or Oscars are doing what they are doing. I would however test your water for ammonia, nitrite, nitrates and pH. Sometimes fish can behave oddly of the water conditions are not right.