I'm not sure about the JavaFX version, but Canvas objects in Delphi (where I have to work with them) are essentially just an image file. It has no child nodes or objects. All the "controls" (which are just part of the one large image) within it are all virtualized and use a reference file. I can't extract anything much useful from the Canvas itself.
I ended up having to use OCR to do some basic checks against it. But, thankfully, I don't have to interact with it too much.
Maybe it's different for JavaFX ones. But they do still appear to be drawn objects.
The support article for Java FX doesn't mention Canvas as being a class supported by TestComplete, so if it is an image, you may not get much more out of it.
https://support.smartbear.com/testcomplete/docs/general-info/supported-technologies/controls/javafx.html#supported-classes