Yikes! Mealy bugs indeed! That's quite the infestation you've got going there; it will be difficult to get rid of. First thing to do is isolate this plant from the rest right away. It's quite likely that it has spread to other plants, but you can keep them from getting as badly infested as this one. There's a wealth of information on here about dealing with these guys. For a plant as badly infested as the one in the photo, I would remove it from soil and go at it with a treatment solution and a paintbrush in the sink, to get in every crevice. You will need multiple treatments to get rid of all of them. Good luck!

Sorry for the late reply. But from experience (this same looking thing happened to my Capers plant and a bit on the avocado) it looks like a type of caterpillar. did you look on the underside of the leaves? On my capers it was almost over night that a few of them chewed literally every single leaf and left me a mangled unhealthy plant to nurse back to health... Hope the problem has resolved. Any updates?

I do not recognize this so I Googled it. This might be helpful https://www.google.com/search?q=black+spots+citrus+leaves&biw=1680&bih=857&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjYltv_39nQAhXCRSYKHXBhDDIQ_AUIBigB#imgrc=3GynaX0KoECnTM%3A

Well "infested" is a subjective term as some of all those are normal neutral residents, some beneficial and a sign of healthy soil with a good organic level, and some will depart simply by the normal amount of disturbing the soil getting the garden ready. Iron phosphate pellets like one gets with products like Slug-Go and similar snail and slug controls will eliminate many more. So just working the soil over well would be the most natural way of controlling them There are "organic" grub killers like Grub Away and Milky Spore work but take time and are usually only recommended if the number exceeds 10 per sq. foot as that would qualify as infestation. Sometimes doing nothing toxic is the best approach. Dave