I maybe able to help if you provide more information

I cultivate it in filtered seawater + F/2 medium in lab conditions and shaking with hand but sometimes we can't shake .. It's enough !!

Dear Malibari, You please try to maintain the nutrient level within the sufficiency range, so that you can  prevent biofilm. This idea worked in our lab scale Photobioreactor, I trust it would supportive in your case too. 

Thank you Mr. Arjun 

If microalgae grow in the surface of the macroalgae to form a biofilm, you may try to use ultrasound remove the microfilm.