WD40 is great for water displacement but a waste of time for unseizing things.
Use a proper penetrating fluid, Plus-Gas, Loctite 'Freeze and Release', even diesel fuel works surprisingly well.
Heat may well be your friend used in conjunction with one of the above.
What looks like a white powder round the end suggests that its seized into the tube.
Take your time & be patient, its unlikely to pop out at the first go. DON'T try & force it by turning the wheel as its possible to damage the helm.
Once its out, a phosphor bronze shotgun cleaning brush is handy to clean out the tube - a 12 bore one fits my Mercury's tubes nicely
Use plenty of lubricant when you re-fit the steering cable.