The algal contamination should not be coming from the media because you autoclave it. It should come either from the environment or the seedlings itself. Algal spores are very resistant if the algal spores are in the air, that would be quite difficult to get rid of them. Algal growth appears very fast if there's lot of light going to the medium. With my hydroponics I use dark colour tanks and minimize the amount of light which penetrates to the medium. I'm not sure whether you will able to do it with your hydroponic setup. However you could try to soak the seedlings in a mild beach solution before placing those in hydroponic system in order to get rid of any contamination.

Hi,

You should cover your vessels in a way that light cannot get in the solution or you will always have algal growth. So light is the factor you have to eliminate.

Svetlana Boycheva Woltering and Bhagya Dissanayake ..thank you so much. I'll try to cover my tanks. Lets see what will happen...

Thanks again.

https://thehydroponicsplanet.com/how-to-get-rid-of-algae-in-hydroponics-for-good/