Hello wonderful and helpful members. I don't know if I'm doing something wrong or if there is something attacking my meyer lemon tree. Could you please share your opinions?

I found this leaf on the ground:

So I sprayed the tree with Neem Oil Extract. I have also noticed two other leaves with some discolor. You can barely see it, so I hope whatever it is I can solve it before it's too late. The tree is in a 5-1-1 mix. I water and feed it with Foliage-Pro when it gets dry, around 8-10 days