I think its only something you spilled onto the leave. Or a waterdrop that is burned by the light into the leave. Dont worry, as long as it does not spread.
If you spary your Plant, do it before the lights turn off, to avoid those burnings
Could be a sign of over watering.
This happen to me a while ago. I got a fungus
I kept the plant to wet.
Foiler feeding is great. Just do it at night
First 15 mins lights go out