that's pilea "moon valley". Its a great viv plant, and loves the high humidity. It seems to do better with more light in my experience, and with more light, the top leaves get a pinkish tint to them. I have some in my 20 gallon auratus tank. Its only been there for about 2 months, but has already grown a bunch and even bloomed for me (a cool thing about it is when it blooms, it shoots out puffs of "smoke" when the flowers open). Here is a picture of mine
Hope that helps!
Ryan