We have a 16,000 gallon gunite pool colored with light grey plaster. It is 3 years old and we are located near San Antonio, TX. Last winter we got some black mold in shade areas. We bought a black mold chemical online. It worked great but left lots of white or bleached out specs discoloring the service. It is not scale but discoloration. They are small in size such as 1/4″ but sadly there are lots of them. Any suggestions? Thank you.

It sounds like you added trichlor granules, which are about 90% chlorine, acidic and slow to dissolve.  Your description suggests that the surface was etched and that the gray colorant was destroyed.  If this is the case, it is not reversible.  Most likely the mold/algae grew in that area, because circulation was poor and sanitizing chemicals were not being delivered, to that area.  Dead zones promote growth of algae or mold.  Improving circulation is an important step, in help prevent a recurrence.  The Pool Circulator replaces standard return jet fittings and turns the return flow into a spiraling pattern, which reaches areas throughout the pool.  This delivers sanitizing chemicals and makes algae/mold growth less likely, by improving chemical distribution.  Acid washing the pool, MIGHT soften the color differences.  I hope that this information will be helpful.