Nice rep message

I just noticed a nice rep message in my user CP. From an anonymous person (no name listed). Thanks for that whomever you are. I think that was the right usage of whom. Whatever, thanks dude.

What did the message say? I got one too...

Hey Buzz & Essential,
I can see the rep posters in my user cp; they're not anonymous. You can't the posters?

P.S. Buzz, that was indeed the right usage of "whom".

Can someone please help me understand this?

I have received reps from 4 members. Of the 4, I can see all their names, but only one has a green colored icon on the left side. All the others have a gray icon. Does this gray icon indicate a negative rep? I would expect negative to be red color. The comments are certainly not negative. One of the gray ones is actually a thank you message.

Do members have to have a certain number of posts before they can give positive reps?