Answer:

It is probably an algae known as black algae. It can be hard to treat as several of the questions/responses on this forum indicate. The water may or may not be safe. Obviously algae is surviving in the water, so what else might also be surviving there? I would at least recommend testing chlorine levels to be sure there is some free chlorine present in the water. Black algae can be very stubborn, so the chlorine alone may not kill it. However, just because chlorine is present does not mean there are no other microorganisms that can cause illnesses to swimmers. Bottom line is if there is free chlorine present, the risk getting sick while swimming is much lower, but there is still some risk from other chlorine resistant microorganisms.