cause of death?

I understand this is a bit of a longshot, but thought I'd post here anyway.

came home from work today to find one of the cherries dead. the others seem fine, and a test on the water showed that all the parameters are normal. I can't see anything on the shrimp (like infection or injury).

believe he was a black rose, but he's discoloured a bit now.

any ideas what it could be?

Things happen.
I don't think anyone can give you a perfect answer.