algae releases oxygen, but then consumes the oxygen again latter when the energy is needed. There is just a surplus that isn’t consumes - everything that makes up the cell isn’t recombined, also oils and such may result as well. Sometimes this surplus rots/burns thus converting back to CO2, but other things can happen (sink to the bottom of the sea where there isn’t spare oxygen to recombine with). Not all recombining is perfect either, sometimes you get coal as a result. The vast majority is recombined, but there is a lot of algae and so just a little bit of difference makes for a massive amount that isn’t. (not near enough to make up for all the fuel humans are burning - but overall the earth seems to have been slightly net negative in CO2 before humans started burning fossil fuels for energy)