This is Slightly underpowered. The ablility of giving your opponent a free mana crystal to use immediately next turn is pretty bad for you actually, especially factoring in dispel, transform, and just plain old not being able to trade this in fast enough.
Mali wisp has very little room for balance, because everything about its design is either integral or it doesnt matter in the slightest.
Because playing mali aggressively is like playing a 4/3 for 4 mana, you should really almost always play it defensively, despite objections. This renders attack useless, and the hp is no tas important because a competent person will always body block for wisp, keeping it out of general reach, and too much removal is not damage based, although hp is more important than attack by far. Mana cost changes make it too op or useless for more in depth reasons I dont care to explain rn, and changes to the effect just change the card fundamentally or making it worse to the point of unplayable.
Because duelyst has a board with body blocking, stat changes are inherently much more difficult to use as balance tools. Either because it makes a card too op, or too weak, or doesnt effect it. This is similar in other card games, but much more evident and amplified in duelyst