I've grown Plumeria for 30+ years & never seen a 'growth' as per the pic attached. It's been like this more than 2 months. Growing slowly now about 4" across the two "parts". It's not a leaf & is fairly hard to the touch. This particular plant I transplanted from a cutting 7years ago. I'm in SoCal. Anyone any ideas???

That is a 2-horned fruit. Looks like pollination took place. When the fruit is ripe, each horn will split open along a longitudinal line to release flat seeds like this.



If you want to harvest the seeds, you can bag the fruit now or just keep a close eye on it and take action as soon as you see it break open (but before the seeds have had a chance to drift away).

You can put a pice if panty hose, or a stocking over it to keep the pod from splitting open. That way is easier to harvest the seeds when it is ready. Good luck!

Many thanks guys.
Very interesting....