I have a burgundy coloured acer and this year only half of the tree is in leaf. Last year I noted some pale green crusts on branches. Can it be pruned back or is there anything I can do to bring it back to health?
Regarding Acer Palmatum
This article mentions various points of Japanese maple care that you might want to check on: https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maple-care.htm
This article covers leaf scorch, which can result in leaf loss on one side of the tree: https://extension.missouri.edu/p/G6881
In order to find out what is affecting your tree, you should seek the advice of a horticulturist or university botany dept.
Pale green crusts on the branches sounds like lichens, which are not harmful. This article tells more about them: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm