Unwell Yellow wrasse, advice?

[Prodd]

I have a yellow wrasse that although shy has been fine up until recently. He disappeared into the rock work for around two days, then this morning he appeared lying on his side. he seems sluggish and seems to 'rest' often. he then dips a couple of times into the substrate and the process starts again. he seems to go for food, although doesn't eat, or eats very little.
All peramaters are fine, all other fish are fine. He isn't bullied and has no marks, blemishes, patches, spots, etc. He hasn't even lost colour.

Any ideas?

 

[Prodd]

Since the earlier post the wrasse went into hiding for the day, has only come out recently and has lost colour on his underside.
Still brightly coloured on the top side, now just grey along his belly.