Nxte, if you cant see any mite under the leaf, right up tight to the under veins, than you dont have white mite ok, in addition, mite attack the whole plant top to bottom, your only seeing the lower leaves affected.
I take it it is the first time you have used cocco ?? I have seen this age old issue many times Nxte, that is, your cocco is still very new, when new, it is very alkaline in it's content. As cocco fibers hold a large alkaline reservoir of calcium 'Alkali" salts, bound int to the "New" fiber's..........
To prove this, do a 'Run Off" test, watering the cocco with a plain water (Watering), Ph'ed to 6. Then get a 100-150ml of fresh run off water, and "Take The Ph".............. you will find it is quite high, around 7 to 7.5'ish....
The first nute to be locked out to high Ph is; Phos then iron & manganese...
View attachment 1017194
it is my opinion, that the slight, very slight, yellow micro spotting, is the early sign of Phosphorus deficiency developing. It is only in the bottom leaves as, these are the younger leaves, when the cocco was first wetted, first 1-2wks.......... when the Ph of the new cocco is very high................. this is why House & garden, now pre wash their cocco to reduce, high alkalinity, that the new plants dont like !!......... ALR
Addendum;
(Advice)
Bin the cocco !!....you want trouble free medium, Ph 6.5 stable start to finish ??......... gold label special mix
Now you have started, you should finish in cocco........... however, as they are young, you could to prove a point, get some gold label special mix, tap out one of the plants, re pot it up in same pot with special mix, for a comparison on the yield,
see which one cocco or special mix gives the best result, a re pot of new medium (Same temp as cocco) wont hurt at all at this age, the one plant re pot will shrug it off, no bother............ make sure your feed water is 21c to 23c warm before you feed, or at least above your soil temperature, for best nutrient uptake............... ALR