Citrus Issues

It appears that there are at least 2 things going on here: 1 – Your tree is suffering from a lack of nutrients. We suggest that you invest in a fertilizer formulated especially for Citrus and follow the label directions. When a tree is suffering stress from environmental causes (lack of water, lack of nutrients) it weakens the tree and it is not able to fight off insect and disease pests. 2 – WE cannot see the “white stuff” clearly enough to make a positive ID but it appears to be either a mealybug or a scale insect. For a more positive ID on both, we would suggest taking a bagged sample into your local independent garden center and/or your local Cooperative Extension office. They might also be able to suggest solutions to you specifically for your particular situation. These links might be helpful:
http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/citrus.html
http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/ENVIRON/mineraldef.html