Weird brown stuff on leaf?

A few days ago I checked on my Northern Lights X Big Bud seedling and I noticed some brown stuff on the leaf. I tried pouring a little water over it, tried rubbing it off, tried spritzing it off, but to no avail. New growth hasn't had the same problem and my Blue Mammoth doesn't have anything on it and neither does the mint I have growing in the same area.

are you feeding this embryo of a plant?

Nope, just w/e fox farm ocean forest has in it.

that can be a lot for a little guy. its nute burn, i betcha. i've seen a lot of folks have trouble with seedlings in that mix.

So should I flush or just leave it be since I don't see any issues on new growth?

And yea I heard the same thing.

ride it out, cowboy. i think it'll settle in. if it appears on new growth.......re-pot to like....Light Warrior, or any neutral mix.

Ok thanks, +rep

Er, just in case I need to repot, what would a name brand neutral mix soil be so I know what to look for? I know for a fact that the stores in my area don't have Light Warrior.

any garden center's grower's mix. no nutes if the key. i use peat and perlite with a pinch of lime for my young plants. then when plant is bigger, it'll be able to take the hot soil.