Yes, assuming the research is correct, this would seem like the sort of thing that should be mandated.
Might be better to do it with a pollution tax. Punish losers but don't mandate a winner, you know.
Is there already a suitable supply chain for that to happen?
Algae grows ridiculously easily - If there was a market for this (likely one driven by mandates to eliminate methane), you could have thousands of tons of algae within months and millions of tons within years.
It's a staple of coastal people's diets the world over. There's half an aisle of it at the local Seattle-area groceries.
Also, I don't think algae and seaweed is the same thing?