5VE37

I decided its time to stop playing Creative Mode and made a new game in Survival. I've been playing for a few weeks without a sight of something unnatural.

One time,I went to explore on my horse,then I saw a woodland mansion.

I thought,"Wow,a mansion"

I grabbed my dianond sword and placed my hore on a wood fence.

I went in the mansion and in the chat,there was this player in my singleplayer server.

<5VE37> Why are you here?

I was suprised another person was here.

 ???

<5VE37> Your not supposed to be here.

 Wat????

I saw the player,he had a Steve skin.

<5VE37> Leave!

<5VE37> Or i'll...

 Well get out of my server!

I tried to hit him but it had no effect.

<5VE37> 298393Leave293820

 NO

<5VE37> Goodbye!

The game crashed,I checked if the world was there,but it was glitched and was titled: Don't Come Back.

Thats when I didn't play Minecraft survival anymore.I countinued to play my Creative worlds,they worked fine.

I switched the characters in "5VE37" and it sounded like "573VE" Which was like "Steve" but with numbers.

Who was this "573VE"?How did he get in my world?How did he crash the game?