Unless it's oil or diesel sludge, no reason not to
discharge it overboard...it's just primordial ooze. Regardless of which it is, disable your bilge pump and put a STRONG solution of detergent and water in the soup and let it stand a while--like overnight--agitating it occasionally to emulsify it. Scrub to get as much loose as possible. (You may have to do this more than once if the problem is severe)If it's oil/diesel, use a shop vac and dispose of it in the location provided for oil etc by your marina. If it's not oil, turn on the bilge pump and rinse with a hose or pressure washer till your bilge is clean and the water running out is clean enough to drink.And in the future, don't wait till stuff hardens in the bilge to remove it!