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Use langsamer for "slower" when it's a predicative adjective (roughly: when it's after the verb "to be"), because predicative adjectives don't take gender/number/case endings.
Or when you say "more slowly", as adverbs are often also formed by using the bare adjective stem.
Because of the 's that you used.
Contractions involving pronouns are generally accepted automatically (e.g. if the maintainers wrote a sentence with "it's", then "it is" will be accepted as well, and vice versa), but contractions involving nouns are not automatically accepted -- maintainers would have to add each such variation by hand.
I recommend that you do not use contractions with nouns on Duolingo -- write them out, e.g. "A snail is..." rather than "A snail's...":