Photo by Pat McMahan from http://www.agateswithinclusions.com/images/US/ore/greenmossagate.jpg

The name moss agate is a bit of a misnomer, because they contain no moss. Moss agates have green inclusions that look like moss or seaweed. The green color comes from manganese oxide or other green minerals. It is the most common type of agate inclusion. Moss agates are most commonly found in Montana and Wyoming.  The one pictured is from Maury Mountain, Oregon.
Source: The Gemrock (IGAMS newsletter), 7/2014