It should be:
PHP Code:
$mysidia->db->update_decrease("owned_adoptables", array("mood"), 2, "isfrozen = no");
Or if you're subtracting multiple things, but only if the pet isn't frozen, you can do this:
PHP Code:
if($adopt->isFrozen){
$mysidia->db->update_decrease("owned_adoptables", array("mood"), 2)
//other stuff you might want to subtract can go here...
};