I am not sure what it is either but it does not seem to be spreading to the rest of the plant. I do not think I would worry about it unless it starts to spread to the rest of the leaves. If it does please post some more pics thanks. I see a light sandy color on the leaf is that what you are referring to?
Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
It does look like thrips infestation, but it's not serious. It's really hard to get rid of these tiny little f.....s, but they usually stop bothering my plants when they enter middle flowering stage. Resin as a natural repellent I'm thinking. Check before the lights go on if you have tiny tiny orange larvae anywhere on your leaves. If you do, that's them. They do not like strong sun very much - sorry, I'm an outdoor guy - and they usually show up in the morning and evening (night too probably). Good luck.