Algae slurry diesel would obviate many of the problems with biodiesel from algae since fuel production would simplify to algae growth of any species capable of producing high wet biomass (although if grown in a salt solution, it may need to be washed to lower the ash content).
A prize would help overcome the initial obstacles, many simply perceptual, to this potential. The criteria for the award could be equally easy to state:
The first vehicle, manned or unmanned, of any size that can travel for 1 mile, powered solely by algae slurry diesel.
PS: If "algae slurry diesel" sounds outrageous to you, try looking up information on coal slurry diesel.