Unless the “baked-on grime” keeps the blocks or the sprue plate from closing right, ignore it and keep on casting.
Keep mercury away from aluminum! The only reason aluminum metal exists is because it forms a thin, hard, invisible coat of aluminum oxide on its surface when exposed to air. This coating protects the rest of the aluminum.
Mercury will amalgamate in the surface of the aluminum, bringing fresh aluminum to the surface to be oxidized. The oxide, which would normally stay on the surface, will slough off, allowing fresh aluminum to be oxidized.
I read up on this after an ill-starred attempt to get mercury out of mercury batteries. I ran the paste through an aluminum funnel at one point, and then occupied my attentions to further separation attempts. After about an hour, I happened to glance at the funnel, and saw that it was full of white “feathers” and too hot to pick up with my hands.
After frantic washing, oiling, and rechecking to make sure the reaction had stopped, I saw that the inside of the funnel had been etched and pitted. Didn’t ruin the funnel, but it would have messed up a mould cavity.
Mercury doesn’t amalgamate iron or steel, so you can use it to get lead off that. But don't try it with brass or aluminum.