The spotted leaves look like they are showing signs of rust, a fungal disease that is relatively common to Calamagrostis (and a number of other OG's as well). It often appears in spring, brought on by warm, damp weather, but it can appear in summer as well especially if overly wet/rainy or with high humidity and crowded plantings. It's not that big a deal and can be treated successfully just by cutting back and destroying this season's foliage. I might be inclined to do that sooner as opposed to later just to keep the disease from spreading to other grasses/plants.

It's hard to control the weather but avoiding excessive irrigation - especially overhead - and providing sufficient air circulation should reduce the problem going forward.