After using the LEGO a few times to make molds, they do get dirty. Some of the clay sticks. Some of the silicone gets into the inside. Plus you have the sealer mold release that leave traces as well. How do you clean it up?
In regards to the silicone, you basically rub and pick it out. For the clay, sealer and mold release, your best bet is to just wash it.
I have heard stories from others about various methods of washing LEGO. The one that sticks out the most is when someone put their LEGO through the dishwasher. Even tho LEGO is injection molded, it is still plastic. At about 170 F it starts to melt. 212 F is the boiling point of water. And if steam is achieved, it can get reach even higher temperatures. If you use boiling hot water, you will warp all your LEGO.
The best way is to hand wash them in warm soapy water.