yeah if its just every once in a while then thats fine but if its fairly often then it sounds like ich, they can display this scratching behavior for days before the cysts arrive, the white cysts that you see are just the eggs and the rest of the ich lifecycles are invisible in the water, raise the temp to 86 and add 1tsp per gallon of salt today and another tsp per gallon tomorrow and keep the dosage of 2tsp per gallon for 2 weeks just to be sure its all gone (make sure to replace any salt you take out with water changes), doesnt have to be specifically aquarium salt, just table salt that doesnt have iron or yellow prussiate in it. def need to get that fish into a larger tank asap, with that large an oscar in that size tank you need to be doing like daily 50% water changes as she is probably already being stunted, and just so you know that 50g is great for now but she will need a 75g to herself when grown to be comfortable
