My 5 year old large acer has got a orange coloured growth on the trunk and some branches, and it appears to be killing it. Is there anything I can do to save it?
From this angle, it looks like some kind of scale. If it is scale, you will be able to scrape the little yellow things off with your fingernail. There are several types that can attack Japanese maple. These articles have a lot of information on the pest: https://www.gardeningknowhow.com/plant-problems/pests/insects/control-plant-scale.htm http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7408.html.
Of course, the best thing to do is to get an identification from an expert, who will also be able to advise you on the best treatment available to you where you live. You can call up a certified arborist.
If you're in the US, you can access the Extension Service in your area https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm
In the UK you have the Master Gardeners: http://mastergardeners.org.uk/get-involved/find-your-nearest-master-gardener/
Elsewhere, you can find the Master Gardeners, or a similar organization, through any botanical garden.