One of two things. First off you either have a slug/snail making your new Opuntia pad a meal. Tbh this is what’s probably going on with your plant. I would bet the farm if you went out and checked your plant at around 11pm you would see slugs;).

The second could be marking from fast growth. I’ve had this happen on Opuntia. In the winter the plant will go from dehydrated to hydrated fast from the rain, and the plant will get what looks like “stretch marks”

I actually did see a rainbow, luminescent slug trail on one of the pads. I am sincerely hoping my plant can grow out of this and persevere; do you think it’s possible?

The pad will just have scarring overtime, not a big deal. Keep your eye on it because you don’t want those little molluscs to keep making a meal out of your cactus. If they do you may want to get some Sluggo;). I hate killing things, but those darn slugs/snails can destroy plants!