Washing the helmet pads is a good idea.

But it's also possible it's not acne at all. I get dyshidrosis in humid weather, which is a form of eczema (also very stress related). I particularly get it on my hands under gloves, and on my forehead under my helmet. It appears as little fluid-filled blisters on my hands, or tiny pimples on my forehead.

Keeping the area as dry as possible helps, and steroid creams as needed - but be careful with steroids on your face.