It uses these text strings that are in your language file...
The rtf_ strings are in RecentForumTopics.xxx.php
$txt['rft_name']
$txt['rft_to']
$txt['started_by']
$txt['rft_in_Board']
But that will changes the strings also on desktop view. I personally would suppress the started by and in Board strings on mobile.
In your theme you can do that by updating the file RecentForumTopics.template.php
Find
echo'<th scope="col" class="lefttext ',$class,'" >', $txt['rft_name'], ' <span style="color: #'.$modSettings['rft_color_viewnumbers'].';">« ', $counter = $_REQUEST['start'] + 1 ,' '.$txt['rft_to'].' ', $sum['counter'], ' »</span> / ', $txt['started_by'], ' / ', $txt['rft_in_Board'], '</th>';
}
and change that to:
echo'<th scope="col" class="lefttext ',$class,'" >', $txt['rft_name'], ' <span style="color: #'.$modSettings['rft_color_viewnumbers'].';">« ', $counter = $_REQUEST['start'] + 1 ,' '.$txt['rft_to'].' ', $sum['counter'], ' »</span><span class="phoneNone"> / ', $txt['started_by'], ' / ', $txt['rft_in_Board'], '</span></th>';
}