Wipe it dry and sleep well. The HK "Hostile Environment" finish is one of the toughest out there. A little moisture won't make a dent. And neither will a glass of rain water.
I tried this little, unscientific experiment on my Glock 43X. As it's known, Glock went with a new finish on their Gen 5 pistols, called nDLC (Nitride Diamondlike Coating). There are countless online comments, videos, posts, etc., bashing it, as it seems to wear out where the slide touches the holster. Mine was no exception, and after a while you could see some "bare metal" spots, as shown below.
So, I decided to "stress test" those spots in one of the most hostile environments there are...at driving distance, that is: Salt Water fom the Atlantic Ocean, collected in a plastic bottle.
And, on with the experiment ...
Gotta make sure she got wet ...
Really wet ...
And to a drawer she went, for 48 hours. I ended the "torture", wiped it dry and it looked like this... (There was a tiny brown spot where the slide touches the frame, but I lost that picture). Anyhow, you get the idea.
Your HK45 slide stop will turn "redish" at some point. Don't lose sleep over it; that red color finish is the real deal. Keep shooting it...