Short answer
It's not an emergency, but mouldy fruit can cause stomach upset. Watch your dog for vomiting, diarrhea, or a loss of appetite over the next 24 hours. Offer fresh water and bland food if they seem fine, but call your own vet if you notice any worrying signs.
Guava — Food safety facts
Safe to share · Dogs · full breakdown on the linked food page
Ripe flesh is safe in small amounts. The hard seeds can upset a sensitive stomach.
The full verdict, portion guidance and serving notes are collected on the Guava food facts page.