Thoughts about light green leaves subtle light spots?

It doesn't look isolated. Some faint spots that look like they could progress to that on the other leaft adjacent in picture too. Could wait for it to get slightly worse to be certain. Make sure your ferts have a Ca component. In this case your other leaves aren't dark, so it could be a real Ca-deficiency. (p/k tox shows this too, but will also have dark/glossy leaves and other issues present) Did that 3-blade leaf recover from damage in earlier pictures? could be that too. I'd wait and see, but likely need to bump Ca a tad. if accompanied by any interveinal yellowing, it's Mg-deficiency. You can solve that with Epsom Salt (Magnesium-sulfate) at about 1g per gallon concentration for a healthy ppm -- but depends on what else you feed. This is the danger of supplemental stuff. You can rarely add just 1 thing easily. I know people are insistent hydro should be 5.5-6.0, but you'll get better use of Ca at a higher pH. Try 6.2-6.3 and see what happens while doing nothing else... i bet it perks up. if you have a complete hydro feed, this last suggestion is definitely the way to go. Coco is hydro, i run at 6.5 because i see all sorts of false nutrient deficiencies and other issues.