The only way you can get hacked by forwarding PocketMine to that port is over an exploit in PocketMine (or something lower level), just don't run PocketMine as root and you'll be fine.
By visiting forums.pmmp.io, you have the danger of PMMP having a script that exploits some vulnerable website that you have previously visited and hack your data there. You are already hacked.
Port forwarding itself does not introduce any danger in general. At least, any vulnerability that is exposed when you open ports is also exposed when you connect to a public WiFi.