Apple seeds contain amygdalin, which turns into cyanide when digested.
Apple stems and leaves should also be avoided.
Too much fiber can cause stomach upset.
Suggestions
Feed only small pieces of peeled apple occasionally.
Avoid seeds, stems, and leaves.
Monitor your cat for any signs of gastrointestinal distress.
Note: even if labeled as safe, any food can potentially cause allergies. When introducing a new food, please feed a small amount and observe your cat for any allergic reactions.