I want to ask about mould in our Berkey that we see after few years usage?

Mould is never a good thing to find, especially in your Berkey water filter® System. 
The good news is, mold can be easily removed and prevented. Mould growth is in the upper chamber of the Berkey® System, where the unpurified and raw water enters the system, though in some cases it can pop up in the lower chamber as well 

Removing mold

Remove the Black Berkey ® Purification elements and scrub them with an abrasive pad or toothbrush. Reprime the elements to flush out any old water. The Black® Purification elements are self-sterilizing, which prevents any bacterial growth inside the filter.

To clean the stainless steel housing, wash both the upper and lower chambers with warm water and a few drops of plain bleach that doesn't contain any artificial scents or additives. The bleach will effectively remove the mold and kill off any bacteria in the chambers. After these steps are completed, you can reassemble the system and begin purifying your water again.

Preventing mold

There are two common causes of mold growth. The first is that water has been allowed to sit in the system for too long. 
We recommend changing the water in the system regularly . If you plan to leave town for an extended period of time, empty the system and follow the proper storage procedures for the filters.