Looks like a very typical bobtail squid or cuttlefish egg, and judging by the fact it's on a seagrass blade it belongs to one of the smaller species.
Thanks Worm Girl.
I have seen many cuttlefish in the same area, but what does a bobtail squid look like? Do you happen to have a photo so that I can look out for them? Are they nocturnal?
Bobtails are adorable little squids with round bodies & stubby fins that tend to live buried in the sand during the day or among sheltering seagrass, corals, etc. They come out at night to feed & mate. I don't have a photo to post but a google image search produces about 200 photos of these charismatic cuties.