Capacitive Trimmer - Why is it full of silicone glue?
Many trim caps cannot be cleaned. Murata, for example, recommends against cleaning for many of their products: 



but they do have some (eg. TZB4 Series) with a 'cover film'. 



Recommended procedure after mounting and cleaning: 



In Digikey's photo you can see what looks like silicone (rather than the typical polyimide cover film one would expect from the name).