So i have a 15 mm 350 DT swiss hub, i want to put some grease in the bearings, so i removed the red dust seal, behind that is a kind of metal cover, so i cant see the ball bearings, should i still smear some on it ?

Are you sure it’s a cover and not the cage that holds the balls in place ? Often the cage is flat on one side.
If you’ve got the bearings out, remove the other rubber seal and the cage will look more bumpy.

Around the metal cover is a thin ring keeping it in place with a break about 2-3mm, use a pin where the clip has a gap to pop it off.

The cover is kind off bulky, i tried gentley to pry it off, but could not, so i think i just try to give it a dab of grease 🙂