Is it on the surface of the water or on the surface of what's inside the tank? What sort of lighting (type and wattage and photo period), temp, stocking, and water changes are you currently doing?
If you need to get a leg up on it, the only algae removing product that I would ever use is: API's Algae Destroyer Advanced. Dose it immediately after your water changes and increase changes to 1 per week to get it into control. It will still take several weeks, but this stuff destroys algae and has no ill effect on fish or plants or your tank's biological filtration.
To maintain a stable system you need to find a balance of lighting, and nutrients. CO2 isn't necessarily required, but it can help if the tank is well planted. It helps the plants to out compete the algae under the correct conditions.
Let me know the above questions, and I can provide a better answer to the specific situation.