Inside of your adapter , among many other pieces and parts, can be found numerous types of capacitors. Some of these are made using a liquid electrolytic substance. To give a general idea: think of the manner as is used in common 12v (car) batteries. These devices can fail at any time although it is more likely for some to fail sooner/&/or less predictably than others.
The advices above are not opinions ! For your safety, please replace this accessory before attempting to use your wii! I only wanted to attempt to answer the actual question to the best of my ability with this subsequent answer. :)
In summary, your adapter may very well be serviceable , ( depending on variable factors, it also may not be), but unless it can be fully restored to properly function DO NOT CONTINUE TO USE YOUR WII ADAPTER!