I'm fairly inexperienced with houseplant care, so the answer to this may be quite obvious. I've had a look online, but I still can't identify the white deposit on my opuntia cactus. I was fairly certain that it wasn't some kind of infestation, but looking at photos of scale, I'm not so sure.

The cactus has always been slightly whitish around the joints, but in the past months, it has developed this white deposit on it's middle 'paddles', the lowest paddles are mostly unaffected and highest ones are completely unaffected. The smaller white dots do come off when I rub them with my finger, but the larger areas of whiteness are hard and stay when I rub them or try to wash them away. I'm really perplexed and not sure what has caused this.

Any help would be appreciated!