Under-Water

Need some input please!

What are the signs of Under-Water? is there are any similarity with over water?

I only water my plant with fine sprayer, and just enough to moist the soil, but some of the plants leafs hanging down like it being over water ???

My plant are 21 days from sprout, outdoor pot grow.

Check the soil and see how moist it is a few inches down. It's probably under watering because from what you described you aint watering it properly. You need to soak the pot so the ROOTS actually get water once every few days.