The image you've shared shows what appears to be some kind of growth, swelling, or abnormality on the cat's ear. This could potentially be caused by a variety of issues, such as:
An ear infection (bacterial, fungal, or parasitic)
An allergic reaction or skin irritation
A growth or tumor
Trauma or injury to the ear
The fact that this has developed relatively quickly, within a day, suggests it may be some kind of acute condition rather than a chronic one.
My recommendation would be to take your cat to see a veterinarian as soon as possible. They will be able to examine your cat, determine the underlying cause, and provide the appropriate treatment. Leaving a condition like this untreated could potentially lead to further complications, so it's important to get it checked out by a professional.
Edited by bloxd io 1 day ago