Some people used kelp also and the worms seemed to be OK.

I suggest you store it in a bucket (w. lid on) for several weeks until they're real mushy before feeding it to the worms.

Don't think eel grass would ever get mushy, pretty tough stuff I guess.

I would think to chop it up into 1" segments, and use it as bin filler. Eventually, it would break down, even if it does take a long time. But if I did that, I'd start with it in one bin for a while first, to see what it does.

For example, I have access to a lot of pine needles. We have two giant deodar trees and they are constantly dropping needles. These pine needles are about 1-1 1/2 inches, and make great carbon and offer excellent air exchange in the bin. BUT, they're stiff and prickly. The worms like them just fine, but they do take a long time to fully break down (2-3 years). In the meantime, they make it slightly more difficult turn turn the bin. The bigger problem is if I want to harvest worms out of that bin, it's not fun to be constantly up against pointed ends.

So, I like to use them when I'm setting up a new bin. I pack the bin with the pine needles and some alfalfa pellets, and after 2 weeks, put in the worms. Starting a new bin, I mostly leave it to its own devices for a few months, and that means I won't be harvesting much for a while, either. This way, I get to use some of my needles, get my carbon for free, but I don't have to handle those pointy ends for a nice long time.