If that valve is an Inlet, on number 1,2,7 or 8, there's a good chance it's just coolant that has run into the port when you lifted the inlet manifold off. (Especially if you didn't drain the block.)
Otherwise it's possible that a badly blown head gasket has thrown coolant (as steam) up there, which has then condensed. Carefully check the old gaskets.
The heads don't normally suffer with cracking or porosity, unless someone has tried to "tune" them, as when opening up the ports, it's very easy to get too close to the water jackets.
If you've got P6 heads, I'd take the opportunity to upgrade to SD1 ones anyway, thus eliminating any possible problems that way.