Hey hey, could someone please tell me what this white part of my nigiri was?

I didn’t eat it which I regret because I could’ve liked it. I tried to take a bite but it was a lot more rubbery and chewy than I expected, and the combination of that and not really knowing what it was kinda scared me. Depending on what exactly this is I will maybe try it properly next time I have sushi. But I’d like to know what it is to calm my nerves about it haha. It’s definitely some kind of meat I’m pretty sure but I’ve just never seen this before. I have some food issues but I’m trying to expand my options so I’d like to give it another chance