Ethereal Credits are token used to buy various items from an Ethereal Soul-Trader.
Once the Soul-Trader is summoned, you must kill players or NPCs which are no less than nine-levels below you. For example, if you're a Level 70 player, you will not obtain a credit for killing a Level 60 player or NPC.
If you find that your Soul-Trader is not giving you credits, unsummon him by reusing the item, and then summon him again. This tends to fix the problem.