Because it's a fossil fuel. It has carbon in it.

It can be used as a carbon neutral fuel.

Carbon neutral means that algae fuel does not affect the levels of carbon dioxide in the atmosphere. Algae takes in carbon dioxide as it grows, then, when the fuel is burnt, this same carbon dioxide is released. No increase, just exactly the same amount. This is why biofuel is so much better than fossil fuel which releases carbon that had been hidden underground for millions of years, so adding to the levels in the atmosphere.

A carbon neutral fuel is one that does not add carbon dioxide to the atmosphere. Biofuel and biomass are carbon neutral because they remove CO2 from the atmosphere during their growing season, and they release exactly the same amount of CO2 when they are burnt. So they are carbon neutral.

I suppose because during manufacture and building, some processes produce carbon, also manufacture of the fuel involves ore mining and refining, and production of the fuel rods.