I found some info about it, in "minecraftcreepypasta.wikia.com"
(nicknamed "303") is a creepypasta commonly said to be "the New Herobrine".According to the pasta, a former Mojang employee was fired by Notch (the creator of Minecraft). Now he wants revenge on Mojang and Minecraft players alike. Entity 303 is said to be not just the former employee but a whole team of hackers going by usernames like "303mojang.com303," and "entity.303". If you have any files in your computer that are named anything with "303", then it could be 303's hackers.303 can also hack in to player's worlds and corrupt them. 1/40 players are affected by 303 and his team. Herobrine actually came to warn us about 303 by showing what he would do.303 plans to ruin Minecraft by the end of 2014 or 2015. Signs have been seen in worlds hacked by 303 saying "hjälp" ("help" in Swedish). That means
Herobrine needs our help (or hjälp) to defeat 303. If he is not defeated soon Minecraft will be in his control. (Note: the first 303 sighting
happened on December 10, 2013. The events of that encounter are no known).
So basically a loser who can't do anything in his life so he joins a bunch of losers to harass people. They're all related to 303 stuff... Meh. Weak.
SCP - 173 is funnier.