I can't tell you why just that i've had em, like little beads that appear on the leaves while they're under the lites. Those water beads didn't cause any issues with the leaves or the plants so i didn't worry bout it. I thought of different maybe's of how n why, but none make sense; closest logic I get is the spots simply seeped outta the leaves somehow.
if you are getting water spots trying changing your water. use filtered water or bottled water. also humidity can cause spots to no matter what water you use. but clean water to me is a +