Just a guess, and I’m a new grower.

Maybe nutrient water that splashed on the lower leaves? I wouldn’t worry about it.

Can you please take a couple of still close up pictures. The rusty blotches on lower leaves. Any leaves not nice and green.
Post them so I can have a better look at your plant. A nice overall plant shot.

What light are you using. What power setting, how high above the top of the plant.
How much much of the trio? How much Cal-mag, anything else?