If it's growing on a plant too thick, it can kill the plant. A picture would help... What color is the algae?
If it's blue-ish green, then you have a real problem. BBA is extremely difficult to eliminate. The normal algae tactics will work with enough determination though..
1.Cut down on the light intensity if possible.
2.Shorten the day length by one hour.
3.Add more fast-growing plants (floating plants or stem plants).
4. Dose with a comprehensive liquid fertiliser like flourish comprehensive once a week.
5. Manual removal.
The only time I had problems with BBA and algae in general is when I had an ammonia spike combined with too much light. Plants won't absorb all the ammonia/nitrogen unless it has sufficient amounts of other dominant nutrients.
In the words of Byron, a small amount of algae is normal in a planted tank and unavoidable. You have more than the normal amount, which is caused by an imbalance of the plants' needs: light-nutrients-CO2. Healthy plant growth suppresses algae.
Rather than add to what I call the holy trinity (
) it's much better to take away. The liquid fert is different, since it's only bringing balance. Plants need 3 Macro-nutrients (N-P-K). N is usually plentiful in an average aquarium, but P and K are often lacking, and in soft water you're often lacking micro-nutrients as well.