while visiting my hobie 16 this weekend after long winter brake, I shocked to find long and ugly corrosion damages along the mast.

What should I do to stop it?

Don't leave the mast up, exposed to the weather for months at a time.

Matt,
What's the difference whether the mast is up or the mast is down?

Lightning rod (as you suggested in the other thread) and air pollution. If it's an old mast, the anodizing is probably all scratched up, exposing the aluminum to acid rain.

Bottom line - if you're going to leave your boat for months at a time, take the mast down and put it under a cover (both the mast and boat). It'll be worth a lot more when you go to sell it.