I have never seen this fish before in Singapore. In China, it seems common. It is called “Pang Pi” fish, or Rhodeus /bitterling fish. Although it is not very colorful, it is quite unique due to its relationship to the clam.
As you can see from the video, it likes to hang around the clam/mussel, and it will even lay its eggs inside. Apparently, the clam also deposits its eggs on the “Pang Pi” fish.
Clams are hard to keep, and can be quite tricky for beginners. It is hard to tell if the clam has died or not. (Even an alive clam does not move much. You can see how it moves in the video.) Once the clam has died, it will start decomposing and producing toxins which may harm the entire tank. Also, it is quite common for clams to have parasites and worms in them.