Doesn't look like any Nudi's are on there, so that's good news. The vermetids are a pest, but doubt they would cause the bleaching. I'd keep looking for other issues as the source of the problem.
You did mention / remind me that your tank is in the sun room. I'm wondering if maybe the glass windows acted like a magnigying glass and maybe too much light got concentrated on that small square patch? I realize that's reaching, but you never know.