Update
Over the weekend I tried every which way to get him to eat nori - left it loose in the clip, folded it tight into the clip, tied it to a rock - all to no avail. Stupid fish would not even go in the same half of the tank as the nori

Most of the other fish had a nibble even the wrasse, but the tang kept his distance............and the patches spread
Yesterday I started feeding a mix of Julian Sprung's Sea Veggies and spirulina flake in the mornings. Some of the fish have one less meal a day now as they don't eat ''greens'', but I tend to overfeed so am not too worried.
The good news is that after just 2 days of sea veggies/spirulina, the blotches are nearly all gone.
Thanks for all the help guys
