If it isnt mold,it may be salt deposits, or water hardness or fertilizer residue.

It does build up on my systems too

If it is mold your humidity is to high.
Try a humidity tester.

These guys have the cheapist parts:

http://www.milehighhydroponics.com/

Here is a link that might be useful: buy cheap stuff here

SO HOW DO I GET RID OF THE WHITE DOTS ON THE ROCKWOOL?

If it's mold, splashing some water on it won't change it's appearance. Salt or hard water deposits will "fade" or disappear after getting wet, mold won't.

I cultivate mushrooms as well as veggies and in mycology a good way to get rid of mold is to make a salt paste and smear that on the affected area(s)... No idea what affect the salt could have on your plants though.

I have mold on the top of my rockwool for sure. How harmful is it? and how do you get rid of it?

Do you use a "cleaner" in your water?

Try running H2o2 added to water through your system for 30 min then add your nutrients to that. Look up the correct amount of h2o2 and what kind to use. The h2o2 will stay effective for 3 days so top off res. with the h2o2/water. You will see the effects almost right away if you dont already use a cleaner.

The H2O2 should do the trick for you, but it's also one of those things where more isn't necessarily better. Be aware of the concentration (3%, 12%, 35%) and the proper dilution, as the peroxide can do the same thing to your roots that it does to your skin and may stunt your plants if over-concentrated.

If you are also using mycorrhyzial amendments and/or beneficial microbial innoculants, the peroxide will neutralize them requiring reapplication. I'd recommend mixing your H2O2 sol'n in a spray bottle and using it as a surface application for the affected areas.

If you look on amazon or certain hydro dealers, your can find vinyl covers for the rockwool cubes that fit around the plants stem and can reduce mold and algae formation.

HTH!