Normal for Water Pump Housings?

krazykuda said
Water - alcohol - mineral spirits; are the three main solvents... One of them should help take the 'stain' off the inside of the housing...

Definitely not a stain. Looks to like it was strategically placed to fill some pitting. Brake cleaner did nothing, but I was able to scrape it off exposing the pitting.

There are some very deep pin holes here and there in other places.

How about the slag? Leave it or try to get something in there to grind it down.