You appear to have scale insect Reguy. They are sap suckers and love new citrus growth. If it is only on that branch, just snip it off and eat them. (OK. Don't have to eat them,) Or just run your fingers along and squish them all. The ants will move scale about on the
tree and they will pop up on other new shoots. If the
tree is infested with them, you can spray it with pyrethrum and whiteoil/pest oil. This is a non selective poison though and will kill other good critters too. Next time you fertilise, ensure the fert you use has reasonably high potassium and this will help strengthen the
plant against further attack. Or sprinkle 2:1 of blood and bone meal : sulphate of potassium as a gentle organic citrus fertiliser. Plenty of mulch, consistent watering.
HAve a good look over the
plant to ensure there are no other minor outbreaks and get them early if possible.