In the original release, Slimer does not have any "food" in his stomach. In the "Best of" re-release, Slimer has "food" painted in his stomach. The "Best of" re-release also is a Toys R' Us exclusive.
*Slimed Peter Venkman: The Ghostbusters quickly discovered that ectoplasm can appear in a variety of forms: ranging from disgusting yet harmless green slime to behavior-changing purple goo. As an expert, Dr. Venkman is familiar with all of them.
Clear Slimer: The increasingly gluttonous ghost known as Slimer was originally confined to the Sedgewick Hotel, where he would often intercept guests' room service orders. The first ghost captured by the Ghostbusters, he escaped at the end of the first film. *Source: Back of package