I don't have much experience with feeding them, but I actually was fortunate enough to find a freshwater clam in a local river almost 6 months ago.
I acclimated him and put him in my 55 gallon tank(6 Goldfish
and one -temporarily housed- chinese Hillstream Loach
, and a ton of malaysian trumpet snails)
I don't have much for algae in the water that I'm aware of as it's quite clear, but I do have many plants and a good amount of algae on the glass and decor(I leave it for the snails and fishies to eat)
The clam has been doing good as far as I can tell, he is still alive after all, I don't ever see any signs of him eating, as he's always buried in the sand, but I occasionally find him when moving plants and I pop him somewhere on top of the sand just to be sure he isn't dead, since I wouldn't know how to tell, but he just digs right back into the sand, and as I mentioned I haven't fed anything, and he has been in the tank for at least 4-5 months and seemingly doing well.
I couldn't find too much info on clams in the aquarium when I was looking, other than them being filter feeders or whatever and eating the goodies in the sand, so i wasn't too concerned about it.
Hopefully you have good luck with yours.