It looks like powdery mildew .... I've never had it before so I wouldn't be 100 percent certain on that but I don't think it slows growth it just makes it unsmokeable due to spores ..... but then it does look like residue left over from salts .... plant is around the average size for around 40 days from seed I know some people show monsters at that age but I don't beleive majority of them I've never grown a plant much bigger than that at around the same mark and if it's been transplanted into a bigger pot you might get slow growth while it adjusts to its new home happy growing hopefully someone with more experience will step in after me or get searching in the search bar in the corner