Q.Our very large/mature Eucalyptus tree
The thick bark near the base started coming off this winter and the trunk underneath has what looks like whiteish mound. Is it deceased? The tree is about four stoories high. Thank you
Oh boy... That is an issue! This is a fungal infection, likely, within the tree. Have any branches fallen off that look like they are empty in the center? I also notice that there are a few larval holes. I don't know if they are currently in the tree, or just have been in the past but I do see insect damage as well. This tree may be on its way out, and in need of professional removal if it is close enough to a structure to damage it. I would contact a local arborist in your area and see what they say. They will get a good look at it, and be able to feel the damaged spots as well.
This article will give you some more information: https://www.gardeningknowhow.com/ornamental/trees/eucalyptus/eucalyptus-tree-problems.htm