I should be able to pin it down to genus (at least a good guess) with a bit of digging through books, but I don't remember the name off-hand, and adults might be needed. It is probably in the same group as Cactoblastis (Phycitini), but I'm not certain. There are several species that bore inside the ovaries and green fruits, and later usually down into the plants. They are usually single (one per flower), and not nearly so destructive to the plant as Cactoblastis or Melitara. Even so, they tend to make a mess, and can kill smaller plants.

I don't often see them out of their burrows and about during the day (usually not even at night), but they will wander out if something happens to their home (flooded, an ant crawls in, filled with sap, etc.), or if they decide to find a better part of the plant (more often they just burrow to another part). Even so, this definitely looks like one of the ovary/pad borers (a very common one that I see a lot).

I could probably tell you the species of Opuntia with a bit more to look at.