Have you pics of the whole plant & other leaves? Where on the plant is this leaf, how old is the plant, what nutes are you feeding, what's the ph of your feed, temp, RH
The plants are about 3 months old and I’m feeding them the fox farm trio for for nutrients. My ph range is 6.3-6.5 and is grown in fox farm ocean forest soil, temps with lights off is 72 and on is usually around 80. I see these spots throughout the plant as well as some of the more upper fan leaves.
This might help Rust Spots On Leaves During Flowering (The Causes And How To Fix Them) - Grow Light Info
Check underside of leaves for thrips. Use a hand lens. The nymphs will feed by sucking. Look careful, they are tiny.