It's just the glue coming off.
I've had the same issue twice in the past 6 months and had to RMA the headset each time because they don't sell replacement earcups,
and the only info on replacements that I've seen is a question Corsair posted on here asking what people want for replacement earcups. Then nothing for over 6 months.
Starting to think they aren't going to sell replacements, which is disappointing because I like the headset but the next time it happens I'm most likely returning my pair for a different brand.