Living algae of all types produce oxygen when in sunlight. At night living algae produce carbon dioxide and use up oxygen as part of their metabolic process. Dead algae consumes oxygen and produces carbon dioxide as it decomposes.

So, if it is sunny and the algae is alive it certainly could be oxygen. In any case, a certain amount of algae is inevitable and not cause for concern.

One way to make sure algae doesn't get out of control is to add floating plants, probably next spring. The plants use up phosphates, which can fuel unwanted algal growth.