Since the bulbs are easily 17-18" from the substrate, you definitely have low light.
As Darkblade recommended, you could spot treat the plants with Excel to help get rid of it. Hydrogen Peroxide also works really well.
Since you have low light, you won't want to be dosing tons of fertilizer or lots of Excel. Could you post a photo of your tank so we can see how many plants you've got?
What are your parameters? Temp, hardness, pH, etc?