White stuff on top of rock wool other then mold!!

Ok so i just got legal and started growing i put my plants in a hydroponic drip buvket system... i have it feeding 15 minutes everyhour it looked fine at first then i added my nutrient to the resevoir. the first watering a white substance formed on top of rock wool. I know it would helf if i had pictures but i lost cord to my camera will put them up as soon as i can but until then if you could inform me on what anyone might think it is i would be much appreciative. oh yeah the plants themselves look good healthy and very pretty but then again they are only 2 to 3 inches tall..

Nutrients are mostly made up of various types of salt....If you're running hydro, a little bit of salty residue will be left behind each time it floods. If you don't rinse it off every once and a while it'll build up over time. I've had it happen with hydroton, but have switched to Sun Leaves small rocks. I haven't really noticed it since.

Yep... Just nutrient salts.

thank you all for your help i did figure out it was nutrients.. i was giving it way to much they still look healthy but i have made it the right nutrient measurement now.. i was measuring tablespoons instead of teaspoons. but than you for all of your help. thank you for answering though...