Our cold wet winters are not ideal for Euphorbia. My plant use to get this until I started bringing it inside during the winters. I’ve seen the same marks on other of Euphorbia like trigona. Tbh I‘m not sure if it’s a fungus or frost damage. You could try a fungicide, but I have zero experience in that area. Luckily once the rain stops, which is basically now, the spots will scar up and stop spreading. At least in my experience.

It‘s super hard to keep plants looking perfect when outside in the elements. The summer keep protected from afternoon sun. Next winter keep warm and cozy. I keep mine dry during winter;).