I think I have a hacker on my mincraft.?

I created a new single player world with no mods only John smith texture pack. I was playing on beta 1.5 and I was getting wood. I turned to see summut on a mountain I traveled to the mountain. I saw a default player with no eyes. and in a split second he went I double checked if I was on mulitiplayer but I wasn't. Please can u tell me what it was I deleted the world for caution.