The white sweat is when your horse sweats a lot after a tough training. It becomes white usually because of the friction but it mostly means that he is loosing mineral salt. It is commun for horses that are stressed or emotional.
It can also comes from the fact that he didn't loose his winter hairs soon enough, don't hesitate to groom him more often in order to eliminate the dead hairs. It can also comes from the weight, if your horse is a little overweight, he will sweat more easily and in bigger proportions.
Don't hesitate to shower him after your ride to prevent irritations and spots. Let him walk a long time after your training to recover and check if he drinks enough water.
Is he sweating only after your riding or even on a a slow walk?