Try tracing back those leads in Marketo, and dig through their activity logs. Looks like you have a trigger somewhere which is (or was) setting Interesting Moments when there were no actual values. It's usually "[Type of interesting moment]: [Description of interesting moment]", so these are probably unselected but still being written through a flow step. When those two values are blank, you get a resulting ": " as the Interesting Moment value.
Best,
Edward Unthank | Founder, Etumos