Hello lord...
A type of hair algae. It's generally a sign you're keeping a relatively clean tank. Livebearing fish are good algae eaters. Platys and Swordtails are good, hardy species. They do best in larger tanks, so hopefully this is what you have.
Large, frequent water changes will help reduce algae and if you reduce the amount of food you feed your fish, this will help too. Avoid flaked food that's high in phosphate.
B
Is it only occurring near the surface of do you see it throughout the tank? If it is near the surface it may be due to the lights being on too long or simply too strong. I'd first start with physical removal using a tooth brush. I had a problem with hair algae in my shrimp tank and decreasing the lights plus the addition of floating plants helped deter it.
5-6 is not too long. What kind of lights are you using?
Monte Carlo carpet without CO2? Nice. What is your substrate and fertilizer regime?