I know this is an older post, but in case anyone else has the same problem I am thought I would answer.
It's still safe to drink, but it will taste very sour! You can use it in cooking as vinegar, or add lots of water to it and drink it, but again, the taste is not so good. Making a second batch with the same mushrooms is totally acceptable. As long as it has stayed moist, it should be preserved.
The only times you should worry about reusing a mushroom is if you have grown mold on top. It would be green or white and fuzzy, just like other mold you have seen. Throw it out if it does, and also if it gets fruit flies!