Florida has a number of native mussels, some of which may be confused with green mussels (Perna viridis).  The most common species are listed below, along with some distinguishing characters.
 

Modiolus squamosus photo: Patrick Baker	Modiolus squamosus (=americanus in some texts).  Found in sandy areas of high salinity, seldom in estuaries.  Size 5-10cm.  Shell mostly smooth, but living specimens are partially covered with a mossy periostracum.  Shell brown outside, white inside.  Green mussels (P. viridis) almost always have distinct areas of green on the shell, and both green mussels and brown mussels (P. perna) lack the mossy periostracum.
Geukensia demissa photo: Patrick Baker	Geukensia demissa  "Marsh Mussel" Common in salt marshes, mangroves, pilings, and seawalls.  Occurs over a wide range of salinities.  Size may exceed 10 cm (4 in).  There are many closely-spaced rRidges on exterior, and the shell is shell brown to black on the exterior and white inside.  Green mussels lack the shell ridges.
Brachiodontes spp. photo: Patrick Baker	Brachiodontes spp.  Abundant in estuaries, similar to Ischadium recurvum but usually occurs at higher salinities.  Often grows among oysters or green mussels.  Size 1-2 cm.  Shell with ridges on exterior, brown to black outside, usually purple inside.  Most identification manuals list this as the "scorched mussel", B. exustus, but ongoing research suggests that there are two or more similar species in Florida.  Green mussels lack the shell ridges.
Ishadium recurvum photo: Patrick Baker	Ischadium recurvum “Hooked Mussel” Abundant in estuaries, usually in areas of low salinity.  Often grows among oysters.  Can grow to 5 cm (2 in) but seldom over 2.5 cm.  Shell with ridges on exterior, brown to black outside, purple inside.  Green mussels lack the shell ridges.