Sometimes Slicers seem to have a long memory and list entries that are no longer in the current data set. There is a setting to fix this.
In the image below I have a Pivot on the same page as its data source plus a Slicer for the State column.
If I change the two TAS entries in the rows 7 and 15 to SA and then refresh the Pivot, this is what happens.
So even though TAS is no longer in the data set, it is still shown in the Slicer. Excel remembers!
To force Excel to forget you need to change a Slicer setting.
Right click the Slicer.
Select the last option, Slicer Settings.
In the dialog that opens untick the bottom, right option as per the image below and click OK.
Then the greyed-out option should disappear, as below.