There is not much you could do, really.
Every drive has between 1 and 3 spare tracks which are used when one of the "regular clusters becomes corrupted/unusable. The pointer to said cluster gets redirected to a cluster in the spare tracks.
Just keep an eye on it and if the numbers keep going up, make sure you have a valid backup and be prepared to get a replacement drive.
If the drive is out of warranty you could try to re-partition and re-format the drive. But that will only sometimes cure the situation if it's been caused by weak magnetizing. If you had a head crash etc. those clusters are gone for good.
-DG