create a netwatch for that IP and put on both on-down and on-up this:
/ip fire conn
:foreach idc in=[find where (timeout > 60) and (reply-dst-address ~ "10.254.251.254")] do={
remove [find where .id=$idc]
}
please do not alter the timeout
http://forum.mikrotik.com/t/dual-wan-failover-script-ping-command/150516/1
I do not write the things without reason.
If just one connection on connection tracking is already closed for timeout (or other reasons) during the execution of the clean,
the script stop with error because when try the connection is already closed, and do not finish his works.