The fish either has air trapped in its digestive tract, or has a swim bladder problem.
Stop feeding dry food for a few days and use frozen (but defrosted) instead. If the fish still floats after being fed frozen foods for a week, it has a swim bladder problem. If the fish swims normally after the frozen food, it is air and you will need to reduce the dry food in its diet, or wet the dry food before feeding.
I'm not sure. It looks like a Pristella tetra but they don't have a big black dot on the body. There are images of red phantom tetras on google and a couple resemble your fish but they don't look like the normal red phantom tetras I have kept.
If you can post a picture of it, other members might be able to identify it.