Maybe he/she is just the subdominant ram, they take on paler colours than the dominant ones that are brightly coloured. Does the ram have a black stripe horizontally across their body in the middle? that would mean that they are stressed.
when we first got the pair about a year ago the ram thats lost his black markings was the boss and would chase the other one all round the tank but now the roles have reversed , he/she has lost all its black markings