Turn over the nest to expose its open base and pull out all of the plasticine. Carefully winkle out all the little bits that will try to cling on or hide in tiny cracks. If any large chunks seems really stuck, it might be a good idea to trim some of mould away so that the same doesn't happen with the chocolate later. Give the mould a thorough wash under hot, soapy water.
You can see here that I failed to use a proper release agent, so was left with a heavily stained mould. This required hours of painstaking scrubbing with an old toothbrush to clean fully. Don't make the same mistake: check that your plasticine and release agent are compatible with your silicone before you start the project.