What is this on the leaves and how can I treat it
Same issue as your other post - too blurry, need to know where you live, are trees in-ground, in containers outside, or only inside.
I hope you will get this link. Because this looks like what you have - Wooley Aspids. On the link, it says that you must also have ants.
To get rid of ants forever just throw Dried Molasses all over the yard - found at a Specialty Garden Center. I buy 20 lb bags and apply 1 time a year or less. Not in rain. This stuff is amazing. Changing the soil to work wonders plus the getting rid of ants.
I just bought Spinosad for my citrus. I don't know if this will help aspids, but you can research this. It is organic especially for citrus problems. Check out the link. The pictures at the link are the same as yours.
Here is a link that might be useful: Gardenweb
The thread you linked to was incorrectly diagnosed.
The problem for that tree was woolly whitefly. Not woolly aphids.
Hand crushing works wonder. I find I can take care of a tr at 8 feet tall with an 8 ft spread a lot faster than one would think. It also decrease number to a level that predator insects can catch up with. A powerful water blast works wonders for non flying insects. both take less time than driving to a store, buying the product, documenting the credit card/ check records, balancing your bank account. then doing the extra tax records at years end. Try it it works.