what is this?

what is this???!
here's my set up,

two feminized auto flowering seeds, Crystal meth and skunk #1

Whitney Farms Organic Potting soil, 10% perlite

3 gallon smart pots

General Organic Nutrients: BioThrive Grow, BioThrive bloom, Diamond Black, BioBud, BioWeed, BioMarine, BioRoot, CaMg+

80-87'F

30%-45% humidity

2.5x2.5 space

1 oscillating fan on high

1 evaporative air cooler fan

400 Watt MH w/ wing reflector for veg

600 Watt HPS w/ wing reflector for flowering

moving them to a 3x3x6 tent for flower.

Is that the only leaf with the issue? Other plant too?

You posted the same pic twice.

Increase your cal-mag next time you feed.

^^^ Agreed, looks like a calmag def.