white help: When I iron clothes, sometimes these little white specs come out of the iron on to the clothes. - Help.com

Are you using spray starch?

its probably mineral deposits from the water you are using in your iron. Try using distilled water only. this also increases the life of your iron, tap water has minerals in it that build up inside the iron.

the other thing it could be is the spray starch as mentioned above, but if you are not using that, try switching to distilled water (its not that expensive, a little more or less than a dollar per gallon, depending where you get it).