im pretty sure its save to spray up until harvest. Smoking moldy buds is not the best thing to do so i suggest spray it to be on the safe side. i use neem oil on my plants.
i started with hydroponics for my first time with 2 plants and they started to do the same thing. i got all the nut. out of the water and left them for a little bit and they seem to get better but i was still scared so i switch to soil and now they are doing much much better.