Easy Bud Autoflower

Everything was going smooth until Day13. Something appear on first true leaves. Something that I did not like, because I experienced that on my first plant, but in much, much later state. Something like a corrosion on surface of a metal. Back then I was thinking it's due overheat from the light source. That leaves eventually dried out and new one was also affected as well, but not to many and I just ignore it. This time I flush plant entirely with bottled water! I'm thinking something is wrong with my tap water or I don't know what. Current condition seems to halt at that state and new leaves was not affected. But that's was also wrong...