Should I be worried about this?

These (rust?) rings appeared after sanding down popcorn wall and mudding some holes left over from nails used to hang frames. The room behind this wall is a bathroom. We want to mud the whole wall flat in order to place some wallpaper on it. There’s some light browning next to some tape I laid down as well. Could I continue to sand these areas and place a second coat over it or should I do something else?