Need I repeat myself? Greentech will succeed when and where it’s profitable.The oil giant Exxon Mobil, whose chief executive once mocked alternative energy by referring to ethanol as “moonshine,” is about to venture into biofuels.On Tuesday, Exxon plans to announce an investment of $600 million in producing liquid transportation fuels from algae — organisms in water that range from pond scum to seaweed. The biofuel effort involves a partnership with Synthetic Genomics, a biotechnology company f