Hi all - wondering if you could help me out.

I have three Mexican Plum trees in my backyard in three gallon pots. One of them has tiny white spots on the leaves (pictured below). I've tried rubbing and wetting them and it doesn't seem to be any kind of mold or fungus that comes off. I'm in central Texas / Austin area where it's been super hot (100+ degree days of late), and have been watering every 2-3 days.

Is this normal or something about which I should be concerned? Is it possible overwatering is causing this?

Thanks in advance - any advice would be appreciated.