The HPS floodlights i got have some kind of crystallization on the inside of the glass

It is white, so i suppose it contains aluminum oxide from the reflector. It also has in places stains of rust. It happened probably from a lot of times that water got qaccumulated on the inside of the glass and then dried up by the lamp

How can such layer be cleaned off ?

http://www.lighting-gallery.net/gallery/displayimage.php?pos=-57172

Try initially window cleaner. If that does not remove it try using white vinegar on a cloth, alternating with a solution of baking soda. To remove the rust stains wipe with a cloth soaked in Coca Cola, it will remove the rust. Rinse well with clean water afterwards.