I employ a cocktail of strategies based on my "feel" for the situation. I have never uttered the words "on your left" on an MUP. I often say "I am easing around your left." That has never netted me an unpredictable response. Often people who walk on MUPS behave like people in parks. That is to say they they lose their awareness and tend to meander. Some bicyclists do to. For me the most important thing is to pass with minimum speed disparity.