Other mussels in the area besides Perna viridis (Tampa Bay), include Perna viridis (Trinidad), Perna perna (Texas).  All the mussels from Trinidad (~30) had a distinct green outer margin.  Virtually all the green mussels collected from Tampa Bay had entirely green shells.  The brown mussel here is entirely brown and has a much thicker shell.  The green mussel shells have been very thin in comparison.