How to modify an add_action() inside a loop of core function

Use the filter hook bp_members_signup_error_message

Try:

function signup_error_change( $error_message ) {
     $error_message = str_replace('<div class="error">', '<span class="val-error">', $error_message);
     $error_message = str_replace('</div>', '</span>', $error_message);
     return $error_message;
}
add_filter('bp_members_signup_error_message', 'signup_error_change', 1, 1);