I recently opened up my machined block I got back from the shop I’m pretty sure everything is okay, then again I’ve never rebuilt an engine. Before I put the MLS head gasket on i cleaned the block surface with brake clean and a rag which left white stains. It’s an aluminum block not sure if these stains are an issue. I also noticed a gouge in the block surface.
This should not cause you issue, but there are several things you should do prior to assembly.
First, check the block in the area of the stains by running a fingernail across it. If your fingernail drags across these where you can feel it, then there's an issue. I'd give you a 99.9% probability you won't feel anything, as it looks fine in the images (but you never know).
NOTE: I just noticed in the 3rd pic machine marks in the deck of the block. You'll probably feel these when running your finger nail across them. This is not what I'm talking about above. I'm talking about where you are finding the white splotches.
Next, clean the block faces with acetone. This will clean off any of the residue left behind by the brake cleaner. I'd suspect the cleaner you used was chlorinated, which is not good for aluminum, but isn't going to kill it. Next time try using non-chlorinated versions and you won't (shouldn't) have this issue. Use paper towels as the "rag" to get it clean. The paper fiber from the towel which gets left behind will breakdown over time, so will not be an issue. The fibers left behind by cloth towels won't break down as easily and will help to clog up the works inside the block.
And I don't know if you were going to, but I'd tape everything off after getting the block clean and paint it. This does a couple of things, like makes the block easier to keep clean (a clean engine on the outside allows it to run more efficiently), as well as help you to be able to detect any leaks if you use a lighter colored pain. I typically use a high-temp ceramic type engine paint which will withstand a higher heat than typical house paint. I will clean, prep, primer, and paint. This method makes it a little more durable. Just a suggestion.