I second @berserk007 in the ladybug suggestion if there is indeed a mite problem; Ladybugs, used correctly, are amazing at getting rid of soft-body insect pests. I don't necessarily think that's all that's going on here, though. I'm on the east coast and have no experience with citrus trees, but I highly recommend you try posting this on reddit; there are some really knowledgeable people on the gardening and botany subreddits, for example, and there might be more specific ones for your particular problem.
Edit: Also it really can't hurt to give it a spritz of diluted hydrogen peroxide for the possibility of fungal contamination. If you're worried about the chemicals, cinnamon or really, really strong chamomile is a surprisingly strong alternative.