Plums (pitted) — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Flesh in small pieces is fine. The pit is a choking and cyanide risk.
Short answer
It's likely fine if the plum was truly pitted. The flesh itself is not toxic to cats in a small amount like this. Just watch for mild stomach upset, such as vomiting or diarrhea, over the next day. If you see those signs, a call to your regular vet is a good idea.