First Grow

This is my first time growing and my station I have set up is experimental. Right now I have three plants in a 3x3x3 box. I have reflection inside for the plants and I'm using a 23w fluorescent with 1600 lumens. I went to check on my plant this morning and I found a white/off white residue on top of one of my plants. Not sure if salt from watering or what but I would like to know now what I can do if anything before I take the plant out from the other two

It could be powdery mildew. How are the temps/humidity levels?

My temps are usually always between 75-79 and I'm honestly not sure about the humidity. I have a fan that goes off every 2 hours for circulation purposes

Leave the fan on all the time.
If you are vegging, it is good all around.
If you are flowering, it is good simply for air movement.
More information is always good if you want precise correct answers.

Fans should be on 24/7, plants need constant air circulation, you're choking your plants.