My mom forgot we had a cactus growing in our backyard. It’s also next to a body of saltwater. Any idea what it is?

I asked my mom for a cutting and you can just imagine how that conversation ended. Also, would it be ridiculous to place it in a small 3 inch pot after it dries or should I go for a bigger pot? And ouch, it seemed like it didn't have spines but they were there. Super small, white and almost invisible. Guess who found out the hard way?