20 yr Malus Golden Hornet
20 year old Malus Golden Hornet's trunk has cankerlike raising with silvery slime all over & spreading onto branches. In places there are small white fluff balls. Never had a problem before. Feel it may be slugs/disease/aphid invasion?
Sounds like some kind of fungus, which often manifest in canker-like growths, often slimey, on the trunks. Here's some basic information: https://www.gardeningknowhow.com/ornamental/trees/tgen/cankers-on-trees.htm
To get more specific information on type of infection and treatment, get in touch with the Master Gardener group near you. This link will help you: http://mastergardeners.org.uk/get-involved/find-your-nearest-master-gardener/