Why do I have algae problems and what to do now?

You should have no or very little algae problems with LED lighting, because the light lacks a UV fraction.
In some rare cases, switching to a different type of light can lead to an unattractive build-up of algae. This can happen in particular if there is a great difference between the light, specifically with regard to quality. I could see this happening here.
If you have a dimmer, you could also temporarily reduce the brightness to ensure a more gradual move to LED light. 

Please understand that it is very difficult, if not impossible, to diagnose problems without being there on the spot.
• Water pollution? Phosphate? Nitrate?
• Perhaps the LED lighting is now stronger than the previous lighting system. 
• You are using (too much) fertiliser because the plants are not consuming as much of it under LED light as they did before
• The period of lighting has been changed 
• The light colour is much different to that used before 
  
If you are operating all the LED bars at full power at the same time, I would suggest initially switching 2 of them off (or even all 3) then increasing the quantity of light each week (as long as algae growth reduces). = darkness treatment.