In the listing_price view file, you can replace the line:
echo '<span class="hikashop_product_price_per_unit">'.JText::_('PER_UNIT').'</span>';
by:
if(!empty($this->element->product_id)) echo '<span class="hikashop_product_price_per_unit">'.JText::_('PER_UNIT').'</span>';
else echo '<span class="hikashop_product_price_per_unit">per m2</span>';
and that will do it.
It's really easy to do it only on some pages ; just prefix your CSS with a class or id only used on the page where you want to apply your change.
For example, to change the color of the prices only on the checkout, you can add such CSS:
.hikashop_checkout_page .hikashop_product_price_full{
color: green !important;
}