Help !

So im new to growing and im using a grow box and ive started to notice one of my plants soil on top is slightly turning white ...is the soil just dry or could it be mold if so any tips on how to prevent and cure my plant .. any tips at all would be helpful

Pics would help..

I bet it's sodium build up and not mold.. But I don't know with out seeing it.

What type of soil, water, and nutes are you using?

can it be wiped away easy ...if so maybe mold
but most likely just nute salts

it doesnt rub off easly