One of my mop top trees is unwell, it’s leaves are yellow and the trunk has split and has soil in it
Welcome to the Bunnings Workshop community @kathy01. It's great to have you join us and many thanks for sharing your plant issue.
That soil-like material is quite possibly frass which is a produced by borers. The borer larva hatches from eggs laid inside the plant stem, as it tunnels to the outside of the stem, it ringbarks the inside of the branch, causing the limb to die.
Under normal circumstances, you would expect to see small holes and the frass being pushed out of them. In this case, it is quite possibly a severe infestation. If you rake some of this material out, you might come across the holes and the borer themselves.
Let me mention @Adam_W to see if he can confirm this for us and provide some advice on a course of action.