i would avoid freshwater mussels
i've heard when reproducing they have a larval form that is parasitic. they attach to the gills of fish. (i don't know what happens after that as they grow and mature, or how it affects the fish - never looked into it)
with that in mind freshwater clams are preferred.
the biggest concern i've heard about freshwater clams is they WILL starve to death in a maintained tank. to check if your clams are doing well check their size to see if they're growing. due to their slow metabolism starvation could take from months to a year or so.
to keep them alive the lazier aquarium hobbyist who doesn't take care of his tank is going to have the most success in keeping clams from starving.
personally i would suggest infecting your tank with greenwater to keep your clams alive, if you insist on freshwater mussels, i'm sure the same would provide them with food too.
as to if the clams will clean out all the greenwater then starve, ... good question, as soon as i have my tank ready i'll find out. (this is the route i'll be heading.)
(using sandblasting glass beads for substrate - in case the worst case scenario and one dies, i can find it.)
"You cannot understand a system until you try to change it."