There are lots of ants around, they just don't like to be photographed :)
This is the whole tree:
I'm in Sydney, Australia.
It appears to be scale insects.
I'm not experienced with scales, but I imagine you can use neem oil and/or diatomaceous earth once in a while to help. I would get the diatomaceous earth all over the stems, wherever they are. I hear you can put a circle of cinnamon or something around your pot to help deter ants.