https://www.mass.gov/doc/chapter-4-rules-of-the-road-0/download does list a "pedestrian hybrid beacon (PHB)" with red and yellow lights:

>A Pedestrian Hybrid Beacon (PHB) allows pedestrians to safely cross a roadway. A PHB only operates when activated by a pedestrian. When all lights are dark, you can proceed with caution. When the bottom yellow light is flashing, you must slow down. When the bottom yellow light is solid, you must prepare to stop. When the top two red lights are solid, you must stop for pedestrians. When the top two red lights are flashing, you must stop and proceed with caution if clear.

But "flashing green circle" is not listed. I don't remember ever hearing about those in Driver's Ed and I've never seen one.

The town where I live has pedestrian crossings with a button that turn on blinky yellow LED lights temporarily. Or ones that trigger a traffic light to turn red. Those seem to be the more modern version.

Edit:

http://archive.boston.com/news/local/articles/2008/02/03/lines_lights_and_rule_breakers/?page=1 quotes a Cambridge PD officer (in 2008) saying flashing green is the same as solid green but intended to get people to slow down, typically used when the cross traffic has a flashing red.

I used to live somewhere where some lights would change to flashing yellow along the main street and flashing red on the side streets late at night. But still never seen a flashing green.