I would advise you to mention it, since its on your arm. In qatar airways they do not only ask you if you have birthmarks, scars or tatoos as they also require you to draw it on a paper, describing the shape and lenght of it. In addition, in the Final Interview, they ask you to pull the sleeves of your shirt up, so they can observe both of your arms. In non visible parts of the body i'd still risk, in the arms not really! It'd be pretty bad if they saw such a big mark that was not on the draw, it's not like a bunch of small moles that you forgot or smth! Hope it helps!
