This is my first time growing tomatoes from seeds, and so far everything looks good. However, I have noticed that some of the plants have these yellow spots on them. This is a recent issue and I am becoming worried. Does anyone have any idea what it is and if I should be concerned?

The plants are under fluorescent light for about 18 hours a day and I only water then when the soil gets dry.

Please take at a look at the pictures below.

Thanks in advance!