you are sorta wrong a slenderman trys to kidnap children he does not care about older people he wants children because they are easy to get but he follows you and has 8 arms so he only does it in foggy areas
i think it may be Herobrine because i sorta see white eyes but if you could 1 or 2 pictures of it a little bit closer and move your curser so i could see it better i can tell you what it is. try to kill it
do you have the twighlight forest mod i think it might be one of those poison skeleton thingys wait when ii was typeing this my computer froze and started doing weird things