I agree. And this is something that is going to be repeated many times on a page, which may amplify the effect. It seems to me that the goal is to let people check if someone is a member by simply glancing at their profile picture—but we don’t want to make the indicator so attention-grabbing that it draws the eye away from the content of the thread.
The contrast ratio on the gray one I initially proposed is approximately the same as that on @mentions, and the Maker M has been fit to the pixel grid to optimize edge contrast on standard displays. If allowed by Discourse, I would consider using a tweaked alternative version for high-DPI displays.
Blue was my first thought, but I am concerned that it would affect usability since blue is already used by the Discourse UI to draw attention to key, clickable elements.