so I bought this effects pedal(Zoom G2.1u) without the power adapter, I currently use it through batteries or USB.

the adapter needed is 9v DC,center negative, 300mA.

I found an adapter that is 9v DC center negative but it's 400mA

will that be fine? or I'm gonna blow up my first pedal?

I hesitated to buy the adapter because of the mA, I thought I'd ask here first .
i think it will cuz i use my 600mA on a 400mA pedal (wah pedal)

"I'd rather be hated for who I am, than loved for who I am not." -Kurt Cobain
it will be fine, you can use an adpator with any mA rating as long as its more than the effect needs. pedals only take as much as they need. I use 1330 Ma adapter to power everything via a daisy chain
You can buy an adapter with more current capability, but not one that has less. The 400mA rating means that it has the capacity to provide 400mA, but if your item says it needs 100mA, that's all it will ever draw. In that case, you'd have 300mA that could be used elsewhere. If you were to use an adapter with 50mA capacity, it would overheat and fail.