In this photo it looks like grass between the stones. For a shady area you can try grass seed specially made for dense shade areas

Keep in mind that moss becomes extremely slippery when it's wet in the winter, and will be a slipping hazard.

If you want something between them, I'd go with grass that can be trimmed with a weedeater.

I'm all about low maintenance and it seems like the upkeep of grass between stepping stones would be problematic. Running a mower over the stones would put the mower blades at risk so it seems some manual effort would be required. And I've never had good luck with the dense shade area grasses, perhaps I'm not putting enough effort into it!
Anyway, there are other options for low growing ground cover on this website:

http://www.jeeperscreepersusa.com/Grow-between-flagstones_c_1.html

Pratia was also suggested by a local nursery since this is a very shady area. Also, Irish moss and Corsican Mint. Will any of these be ok to plant in late Sept. in NW Ct. Zone 5? I want to avoid grass since the weedeater would be difficult to use among the adjoining pachysandra.

ispolidogs...thank you for the website. It gives me all the info I have been looking for!