Why is the Raised Zombie glowing green

This is a challenge leftover from the Season 1 challenges. Season one ran from June 5th through October 8, 2013.

Allocate all notable and all keystone passive skills. A green check mark will appear next to skills you have taken. The check mark remains even if the skill is refunded or the character deleted.

Chances are you'll see this type of challenge again once the new race season starts.

Path of Exile Gamepedia

Theory:

In PoE there are two PvP "modes":

• Low-level PvP, with a maximum player level of 28
• High-level PvP, without a maximum player level

You can access the low-level PvP queues from Greust in normal Act 2 or the Event noticeboard in Act 3. The high-level PvP queues can be accessed from the Event noticeboards, which are located in every town (except normal Act 1). You should be able to get into the high-level PvP from the Event Noticeboard in cruel Act 2, but don't expect to win, you will most likely be matched with a player that is level 80 or above.

There are other options to start PvP matches though:

• right click any player and select "Challenge to Duel"
• The Sarn Arena, in the northern Part of the Act 3 town you can find the Sarn Arena, a PvP Arena that can be entered and exited at will

You can find more information on the different PvP modes here.

Reality:

In Cruel your chances of finding another player that want's to PvP are rather slim. Random players in town are seldomly interested in PvP and the Cruel Sarn Arena is usually empty.

Your best bet would probably be to ask for a challenger in global chat.

Alternatively you can create a PvP-only character that starts out at level 28 and can only be used to access the Low-Level PvP queus.