Ram going dark

Hi,
I know a couple of you have Rams (Bolivian / ramirezi) so a quick question.
Two nights ago I noticed that one of my rams looked a bit dark round the
head area. He was eating fine, looked OK maybe a bit thinner than the other
one, but nothing too out of the ordinary. I lost a discus a while back where
it went into hiding went really dark then died, and wondering if this is a
cichlid disease or something else I should be aware of. Any ideas or just
seasonal change?

On a side note, I had a quick look last night before I went to rugby
training, and the healthy one has now gone missing, didn't look too long but
I'll look again tonight, there's alot of places for them to hide.

Cheers

A