While it seems not to bother most horses, it is unsightly and if it's allowed to build up over time, it can cause the hair in the affected area to fall out in clumps.
A horse affected by cannon keratosis whose symptoms are not treated on a regular basis will often have clean-looking legs after a pre-show bath, but the greasiness will reappear during warm-up when dirt sticks to the remainder of the greasy patch.
While not all agree on the cause of cannon kerotosis or on the best way to treat the underlying condition, there are easy ways to keep the build-up of "cannon crud" under control.
Step one is to loosen the crud. This can be done with your fingernails, a scrubbing brush or a rubbery curry or grooming tool. If the build-up is very hard and hair is coming out, back off and wait for the crud to soften before attempting to loosen it.
Once you've loosened the cannon crud somewhat to allow you to clean more deeply, wash the legs with soap or shampoo, applying it directly to the legs. Experiment to find what works best for your horse; an anti-dandruff shampoo could help the underlying condition or plain old dish soap could help to cut through the greasiness. If one treatment doesn't seem to be helping, try something different. While one bath isn't likely to remove the cannon crud completely, you should be able to see a difference.
After washing the legs, dry them by rubbing with a towel. This is a good habit to get into regardless of whether or not your horse has cannon crud because many conditions affecting the skin of the legs thrive in a wet environment.
With some experimentation you should be able to put your horse on a regular leg washing schedule to keep the symptoms of cannon keratosis at bay. Altering your grooming routine to keep that section of hair loosened up will make it much easier for your chosen treatment to penetrate.