add_action( 'bp_before_activity_loop', 'bp_activity_tag_header' );
function bp_activity_tag_header() {
	global $bp, $bp_unfiltered_uri;

	if( $bp->current_component == BP_ACTIVITY_SLUG && $bp->current_action == BP_ACTIVITY_HASHTAGS_SLUG ) {
		echo '<h3>Activity results for #' . $bp_unfiltered_uri[2]. '</h3>';
	}
}