You can add a javascript like
if(document.cookie.indexOf("first_time_visitor=1")<0){
var exdate=new Date();
exdate.setDate(exdate.getDate()+10000);
document.cookie="first_time_visitor=1; expires="+exdate.toUTCString();
alert("Some text to display");
}
to mobile template (actually it's better to make it to be custom html module, because of templates are overwritten at update).