Jared,
you need to find a new pool guy.
Trichlor tabs not in themselves, reason for staining your plaster. There are reasons why we dont recommend using them as a primary source of chlorine, but staining isnt one of them.
The stains could be one of many things.
calcium scaling
metal staining from using algaecides with copper in it
metal staining from using fill water high in iron content
staining from lawn fertilizers or chemicals that may have metals in them
Stains can also be organic in nature.
Here is a couple of tests for you to do that maybe will shed some light on the situation.
take one of our trichlor tabs and rub it for a few minutes directly on a stain. If a stain lightens where youve been rubbing it, then the stain is organic in nature.
Get a bottle of vitamin C pills. Put a few in a sock or wrap in in a handkerchief and put a rock in there with it.
Place the sock on a stain and leave it there till the pills dissolve. If the stain is lighter, then it is metal in nature.
If it isnt organic or metal, then its likely calcium related. No easy test for that, and pics of it would help.
We hear at TFP rely upon accurate and trustworth water test results to try and figure out wwhats going on... that along with knowing which chemicals that folks use to maintain their pools. But i can assure you though, the tabs didnt do it.