Weird spots on Euphorbia leaves
My wife and I have a rather large, nice coral cactus grafted onto a Euphorbia (indoor plant). About 6-8 weeks ago we started to notice some finely textured, small to large, brown coppery spots on the leaves (top and bottom side). It doesn't look like scale to us, but then again we don't have a lot of experience with cacti and succulent diseases (other than an outbreak of mealy bugs that we successfully dealt with last year). None of these brown spots rub off when you touch them.
Any ideas as to what this is? How to get rid of it? Thanks!