I have a spill over fiberglass Spa. It is filled by pool water. A light green stain has developed all around the side of the bottom half of the wall section just above the Spa floor. The rest of the inside of the Spa looks great ... no stains anywhere ... to include the top half of the same stained wall section. My Pool Maintenance Professional states that he uses acid to remove such stains. He is good but I fear the use of acid unless necessary.
How should I attempt to remove the stain?