The following abstract from
http://fins.actwin.com/aquariafaq.html may resolve your problem:
Prophylactics for Algae
Algal spores are everywhere and will always be present in an aquarium unless drastic measures are taken. For fish only tanks, a properly set up ultraviolet sterilizer will kill algal spores in the water and prevent them from gaining a toehold.
For planted tanks, this is not a good solution since the UV light will also oxidize trace elements needed by the plants and will limit the plant's growth potential. Unfortunately, conditions that are good for growing plants are also good for growing algae. Fortunately, plants will usually out-compete algae for the available nutrients. However, if there is an imbalance of nutrients, algae will opportunistically use whatever is not used by the higher order plants. Different algae will utilize different nutrients, causing sporadic outbreaks of new algae types in apparently stable tanks when a temporary imbalance occurs.
An ounce of prevention is worth a pound of cure. To avoid introducing a new algae type to a planted tank with new plants, a simple bleach dip seems to work well. Mix 1 part bleach in 19 parts water and dip the new plant in it for 2 minutes. Immediately rinse the plant in running water, then immerse it water containing a chlorine remover to neutralize any remaining bleach. This will kill the algae and only temporarily slow down a healthy plant. Plants in poor condition may succumb to this treatment, but they probably would not have lasted anyway.