It looks like insect damage, although it could be just showing up from weeks ago. Spider mites are bastards that cause a ton of unseen damage for being so tiny... barely visible. A few weeks after killing them off, I noticed similar markings on some leaves, usually the older ones. It takes the leaves that long to respond to the damage. If not spider mites, some other type of bug. Here is a leaf that is mite free, but they had their way a month earlier and finally I had to remove it. Regarding the nutes… go easy on them until you get the issue worked out as FF soils are pretty hot already and any additions to it can easily cause nutrient overload and the issues that go with it. The damage can mimic some insect damage (such as dried leaf tips and necrotic leaves), so you could have a few things going on. The biggest issue looks like bugs to me, though.
