It's right that 'Martialis' can also be a genitive case, but here, in this verse, 'Martialis' is related to 'toto notus in orbe', more specifically to 'notus' ('known', nom.masc. sg.), so 'Martialis' is nominativus here. It can also be related to 'Hic est quem legis ille, quem requiris' (the 'hic', the 'est' and the 'quem' -twice- are singular, so is 'notus' and 'Martialis').

I hope this makes any sense for you guys!