Mustarda (Μουστάρδα) is mustard, and within the Greek pita and grill world it is a condiment rather than a dish in its own right. It earns an entry because of where it sits in the build: a thin stripe of yellow or Dijon mustard that some grills add to a pita alongside or instead of tzatziki, most often on the meatier, more sausage-leaning wraps. Treating it honestly means describing a component, not a sandwich. There is no bread to assemble and no plate to compose; the question is what the mustard does to a wrap when it goes in.
What it does is provide sharpness and acidity in a build that otherwise leans rich. A Greek grill wrap is warm meat, oil, often fried potato, and a creamy yogurt sauce, and that combination can flatten without something to cut it. A restrained line of mustard adds a vinegary, faintly bitter edge that resets the palate between bites, the same job a pickle or raw onion does elsewhere in the wrap. Good use is a measured stripe drawn down the pita before the meat goes in, so it is distributed rather than pooled, and chosen to match the filling: a mild yellow mustard for a delicate chicken wrap, a sharper Dijon for sausage or pastourma. Sloppy use is a heavy smear that bullies everything else, or mustard added on top of a full load of tzatziki so the two sauces fight and the wrap turns muddy. It is one of those components that is invisible when right and impossible to ignore when wrong.
Its role shifts with the wrap. On a sausage or cured-meat pita it can be the primary sauce, standing in for yogurt entirely; on a standard souvlaki it is a small accent that some shops offer as an option and others never reach for. Many Greek grills do not use it at all, which is itself worth saying plainly: mustard is a regional and personal preference inside the pita rather than a universal element. The yogurt-and-cucumber tzatziki it sometimes replaces, and the cured pastourma it most naturally accompanies, are full subjects that deserve their own articles rather than being crowded in here. As a component on its own, mustarda's entire purpose is contrast: a small sharp counterweight to an otherwise warm and fatty handful.