Why did my positive battery post and terminal turn black?

I replaced the positive terminal about two months ago because it was corroded, and I just now noticed the positive terminal and positive battery post are black. I also noticed there was a stain on the side of the battery. I was hosing down (not water pressured) under the hood the hour before so it might be a water stain but if it was, would it not have evaporated without leaving a stain behind? The stain is still there even after 3 hours have passed, could it be an acid stain? I looked at a picture I took 2 weeks prior, and the stain wasn't there, but the battery post and terminal were black back then too so the terminal and post turning black wasn't caused today.

There was no issue turning on the car 30 minutes after hosing down under the hood. After turning on the car, I let the car idle for 15 minutes to dry off. Once 15 minutes passed, I turned off the car and turned it on several times with 10-20 seconds in between and after the 4th try, it sounded like it had trouble turning on. After the 5th try, it turned on but the dashboard lights immediately turned on when I put it in 4th gear, so I quickly put it in parking and turned it off. I waited a few minutes before attempting to turn it on again and it turns on fine now. Was this caused by a bad terminal connection? And can someone tell me why the positive post and positive terminal are black? Did they turn black because of a bad connection? Finally, is my battery possibly leaking acid? Thanks.