I've been de- fouling my hull, and "tickled" the underside of a toob with one of those paint buffing wheels (the plastic stringy mess type ones) and got it to remove a bit of it.
It was a passing glance with the buffing wheel in the drill , but did make a reasonable job of the small area I tried. I have a variant of it (The halfords one impregnated with silicon carbide) which I'm going to attack the other side of the hull with later this week, and hopefully take half the time to get the stuff off the 'glass. Will try that on the toob too (ha ha, or maybe on a patch just in case it's even more violent than it looks!).
Earlier I did try a bit of coarse grade paper & elbow grease, but it just bounced off so gave up.