If the substance looks like mud, then it's likely a mixture of
oil and coolant, which would normally indicate a blown head gasket. However, a blown head gasket isn't likely to result in liquid leaking out of the engine. Normally, it's from a water port to an
oil port, or from a water port to a cylinder, and is confined within the engine. Have you checked your
oil dipstick to see if there's evidence of coolant in the crank case?
If you do have to change a head gasket, I'd recommend using a Cometic head gasket. Cometic (multi-layered steel) head gaskets are expensive, but they're the very best you'll find. You'll find them in many NHRA Pro Stock engines, and nitrous/blown/turbo Pro Mod engines.