When algae grows it converts carbon dioxide from the air into biomass through the process of photosynthesis. This process directly removes carbon dioxide from the atmosphere.
Crude oil is converted to several products, some of which are combustible and release carbon into the atmosphere and some of which are not used in that way like plastics and asphalt tar.
This means the full cycle of algae growth, conversion to crude oil and respective use by consumers results in a net negative release of carbon into the atmosphere. In contrast the process of removing crude oil from the ground and burning it results in a nearly direct contribution of carbon to the atmosphere because humans are burning carbon far faster than the earth is converting new biomass to crude oil naturally.
