Spotted this while muck diving in Dauin, Negros Oriental, Philippines, in about 10-12m depth.
Appears to be an egg case of presumably a marine gastropod, and was on a fan shaped "leaf" of the alga Avrainvillea erecta, the food plant of Costasiella sp. leaf slugs.
I looked out for others, as I was spotting for leaf slugs, but didn't see any more mysterious egg cases, although I did do alright with the sheep ;-)
Digging online, I found a reference to one similar that was photographed in 2019 in Cozumel in Mexico:
https://gastropods.wordpress.com/2019/09/24/photo-of-the-day-92-eggs-of-cassis-madagascariensis/
That one was identified as the work of Cassis madagascariensis - (Emperor/Queen Helmet Shell/Snail) which only occurs in the tropical Western Atlantic, Gulf of Mexico and Caribbean. (The specific name "madagascarensis" literally means "of Madagascar", but this was a misunderstanding of the type locality by the original author.)
So back to this specimen, I did see a pair of vacant/bashed up helmet, or similar, shells in the seagrass beds near where the eggs were. I didn't think to photograph or get a certain ID, so can't be sure, but the more complete one looked to be a Horned Helmet (C. cornuta) and the other may have been too.
So the Cassidae seem definitely to be present, but which genus/species of them might lay an egg case like this....?
At first I thought it was a Banded Venerid, but it closely matches the features of Japanese Littleneck.
6 live specimens seen over 100m search of shallow subtidal rocks, 10-50cm deep.
Appears to fit the description and habit better than anything else I can find right now, though it's well out of the described range for H. oculata.
Second most abundant green alga in this location, after Ulva.