Can anyone help my cory catfish has this red mark on him just wondering if I should be concerned. Have tested all the normal water parameters and they seem okay. He is the only one out of 8 that has this mark.

To me, that looks like a simple superficial marking/feature pertaining to that particular fish, rather than anything to be alarmed about. If the insides of the gills were inflamed/blood-red in colour, and he was behaving differently or not eating, that would be a different matter entirely. If the situation changes in any way, let us know and we'll advise further. How long have you had him and the rest of the shoal?

Yeah they are all happy and snuffling around. The redness seems to have gone down alot today so must have been as you said. 7 have been in the tank about a month the other came with the tank and he is huge compared to the rest