Problems with my new rose plant

Hello everyone,

Recently I came into possession of a rose plant. I placed it on the corner of my second floor balcony, where it gets plenty of sunshine throughout the day, and i supplied it with lots of fresh soil. A few days in, i noticed white spots and something akin to mildew across the plant. Just today, after closer inspection I also found a little maggot  under one of the leaves infected with the spots. After a cursory search, I found out about the mildew and rose slugs. Does anyone know what these are and how to treat them? My limited experience with plants would greatly appreciate it!