Fresh water SYSTEMS (not just the tank) should be sanitized at the very least once a year but ideally several times. With regular sanitizing with household bleach and filling the FW tank with a potable water source treated with Chlorine you should always have fresh clean water.
To get the algae out that's already in there, open the valve on the bottom of the tank and drain and flush and drain and flush until it's clean. You may need to open the drain valve and take the trailer for a drive on a crooked road with lots of starts and stops also. MOST of them will not empty completely and that dark damp tank is a good growing spot for the problem. For that reason after I return from using the trailer the last thing I do is FILL the FW tank completely with treated water. When it's time to use it again, I drain and fill it with fresh potable water from city water source.
Good luck / Skip