Using seagrass (Posidonia spp) as a high carbon ingredient?

Hey peoples. We live in a coastal region of Southern Australia where we can help ourselves to an abundance of Posidonia australis (seagrass) for free. We've so far found that it makes a wonderful mulch as it seems to be breaking down only very slowly. That's good in a mulch, right?
But is that good in a compost?
When tried it in a Berkley hot pile, substituting it for the usual straw, the pile got extremely hot and stayed hot for most of the 18 days. Which, again is good, right? But it didn't break down even a bit (that our eyes could see).
Nor did the sheep pellets decompose.

We're interested to hear about anyone's experiences of using seagrasses in compost as we'd love to crack the code on this one.
It's such an awesome free abundant resource. We want to use it as cover material for our humanure piles, but we need to know how well it's likely to work?
So yeah, we hope to get some clues here before we give it a go.

No one responded so I will.
A couple of  weeks after posting I found that it did break down and made an excellent soil for a raised veggie bed. Happy ending. Peace! 😊

Hi Bree, I use seagrass (I am in NZ) for my compost piles, generally layering it over the horse manure to keep out flies and smells. It works perfectly, but I use it sparingly so it breaks down well. Generally the heaps break down in about 3 -4 weeks, with turning once or twice a week. I think the trick is to not throw in too much at a time. Happy gardening. One thing I cannot quite work out, is it a "brown" or a "green". When fresh, I am guessing it is a green.