Nopal is the word for green vegetal pads, somewhat like green beans with okra mucus. Tuna is the word for the bright purple or yellow fruit, which has a flavor similar to watermelon.

Oops, yeah I was referring to prickly pears. The bright purple and yellow ones are fantastic. I generally stick with the green ones, since the other two are dangerous as far as stains are concerned, particularly if you're sharing them with young ones!

My in-laws mix both "nopal" and "tuna". Possibly depending on context, but I'm still studying Spanish and nowhere near 'native speaker' level.

I just call them "nopal" as I hear that word more often. Possibly due to the saying "Nopal en la frente" when they tease my sister-in-law her husband.

I can imagine "tuna" being confusing to English-speaking natives :-)