Those aren't hairs - they are insects. They look like long tailed mealy bugs and they suck sap out of the plant tissue. That will cause yellowing and spotting of the foliage.

Try blasting off with a strong stream of water first. If that doesn't work, you can try insecticidal soap or neem oil. If the plant is outdoors, you can also try benefical predatory insect control - lady beetles, green lacewings and parasitic wasps will all gobble up mealy bugs.

Thanks for your reply gardengal48. I am sure these are not long tailed mealy bugs though. Someone from my local organic nursery, my gardener and myself have tried to remove a few to see if they were mealy bugs or other pest, but the hairs are impossible to remove! It almost looks like they are part of the plant, which is really puzzling. I've been spraying neem oil once a week in the evening with no luck yet... I will try insecticidal soap and/or ladybugs next. But I'm almost suspecting some kind of fungus maybe??

The amazing thing is too that at the nursery we checked their jasmine plants and they also had the white hairs under the leaves! So maybe it could be just part of the plant??

I don't think they are part of the plant. Mealy bugs was my first thought also. Dab them with a Q tip dipped in rubbing alcohol. They should come off.

Thanks zzackey. I just dabbed them with alcohol q tip and they did not come off :( ....I did clean up the plants earlier today and found two green leafhoppers though (!). I read online that they transmit bacteria so that could be causing the white and black spots (leaf scorch?). Only thing is that I still can't explain the funny white hairs under the leaves; still a complete mystery.

Any experience battling leafhoppers organically? I sprayed both plants with insecticidal soap after cleaning them, but I think I should get a good natural predator. Any suggestions are truly welcomed!

I would take a few leaves to your local Agricultural center for a positive ID and a remedy.