Apparently these are good to eat and are now found in Florida. There is no bag or size limit and no season.
QuoteDisplay MoreGreen mussels (Perna viridis) are non-native bivalve mollusks which have recently invaded coastal marine waters of Florida, USA. Evidence for the presence of this invasive species has been found on both the Gulf coast from the panhandle to Naples and the northern and north-central portion of the Atlantic coast.
Additional info on the green mussel.
http://greenmussel.ifas.ufl.edu/
FWC Regulations on shellfish.
http://myfwc.com/marine/recreational/recshellfish.htm
Florida Division of Aquaculture
http://www.floridaaquaculture.com/
Here is a link to all of the Florida Shellfish Harvesting Area Maps. Just click on your region and it will show you the approved and prohibited harvesting areas for your area.
http://www.floridaaquaculture.com/seas/seas_shamap.htm
This website also releases advisories on algae blooms and other harmful items, so you should check it regularly if you are going to harvest shellfish. This is from the Aquaculture website.
Because shellfish harvested from polluted water may cause human illness, the sanitary control of the shellfish industry is necessary.
I was told you can still harvest the green mussels outside of the harvesting map and use them for bait and such but it is not recommended to consume them because those waterways and estuaries are not monitored by Florida Aquaculture.