In nature slime is mostly composed of fiber like proteins that are often covered by many glucose groups. Common type of mucus proteins in mammals is mucin and this is also present in fish slime. Mucins can have molecular mass of 1-10 million hydrogen atoms and in addition to being long fibers they also form many sulfur "bridges" between cysteine parts that connect random parts of fibers making it possible to connect several different mucin molecules with sulfur. Having glycose molecules attached makes mucin and other proteins likelier to attract water and avoid degradation by proteases.
One possible use for slime could be in making strong strands although as proteins they probably degrade if bacteria and humidity could touch it. Hagfishes produce lots of slime for self-defense (as shown in Hammonds Miracles of Nature 3 scene). After drying it turns into strong fiber close to the strength of spider silk although with less complex structure.
Prostate secretes some proteins that break up proteins making entire mixture more fluid. For example PSA (prostate-specific antigen) breaks up proteins that kept semen more solid and liquify it for ejaculating.
Slime produced by any body part is likely to smell "fishy" as they share exactly same odor molecule. One receptor that releases slime is acetylcholine and it eventually breaks down to trimethylamine which smells fishy and this could be felt in the mucus/slime of nose, mouth, vagina, penis (due to slime part of semen) and fishes.
