Well, one obvious difference is the medium. Manga is visual, so you can directly see the slime's appearance, the expressions of characters, and the action scenes. For example, in a slime battle scene, the manga can show the slime's movements vividly. However, a light novel uses words to describe all these. It might describe the slime as a gooey, translucent blob that pulsates with a strange energy.
Well, without more context, it's difficult to say exactly. 'Slime' in light novels often refers to a type of creature, which can have various forms and abilities. The '300' might be a volume number, a chapter number, or something else entirely. It could be a story where the slime is the protagonist and there are 300 important elements in the plot, like 300 quests it has to complete or 300 different types of magic it encounters.
Slimes in these novels are often depicted as having a gelatinous, amorphous form. They can usually change shape to some extent, which gives them an advantage in various situations, like squeezing through small spaces.