Without actually seeing the stuff I would say it is a mineral deposit.

Here is a pretty good link concerning this
http://www.applianceaid.com/drain_wash.html#filming

The tan color may be coming from a high iron content in the water.

Couple things to check/do.
Make sure that the heater is working. If you are using a cycle type that does not use a water temperature boost try running with the boost on.
Try a different detergent brand/type. The powder type is still the best overall.
Drain some water out of your hot water tank. Let it run till the water is clear. This will not fix it but may reduce how often you have to clean it.

Try cleaning the unit with vinegar or oxalic acid, sold at appliance stores specifically for cleaning dishwashers.

Since bleach cleans it, I may be totally out to lunch on the mineral deposit idea as usually an acid (vinegar) is required to dissolve and clean deposits.