It looks like the spongebob-health-inspector-nasty-krabby-patty equivalent of an avocado. Probably rotted, tried to sprout, died, and rotted again.

I have a cat just like yours with the same problem and can attest that it really does help. My fluffy guy really was no fan of the process, but he enjoyed summer much more afterwards. There really was an immediate change in his attitude, less apathic, more playful, cuddlier, and just happier overall. Will certainly do it again this year.