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I suspect that you did not clean off the peice of wood or soak it before you added it to your tank? Typically, it is important to do both of these things and perhaps boiling it in water (if you have a pan large enough) would help to ensure that it is sanitary before you put it in the water with your fish.
Your problem could be a couple of things. The two that I can think of right now are that the wood may be 'leaching' into the water and turning it colors. The color could be anywhere from a green to a brown, like coffee, color depending on what the wood had soaked up while in the lake. It would stain the water because a lake is obviously different than an aquarium as well as the fact that you wouldnt be able to tell if it is changing the water's color in a huge body of water, relatively speaking. Otherwise, you could have transported algea on the wood and into your tank or other 'algea promoting' chemicals and you are having an algea bloom right now.
In either case, my feelings for you are to remove the wood and do a water change on the tank. You may need to do a few water changes until the water is completely clear as it was before, but in the meantime, you can get yourself a bucket or other container in which to soak the wood, changing the water frequently, until the water in the container turns clear and stays that way for a couple of days. You should wash the rock the best you can as well before soaking it to help speed up the process.
So, I hope that this at least gets you started. It would probrably help to get a detailed explaination from you on what the wood looks like, what the water color has changed to, and of course, a list of your water parameters. I am sure that by giving us all the most information about the situation as you can will help you get some answers faster. | 
