Hello b...
Your piece of driftwood is leaching tannic acid, but it's essentially harmless. Just do your water changes and eventually the water will clear. This time of year you just need to rinse the piece with a pressure nozzle attached to a garden hose and leave it out in the sun for a couple of days. The heat and sunlight will kill any bacteria. Depending on how dense the driftwood is, boiling can soften it and cracks will develop. In a short time, pieces will break off and float to the top. Mopani wood is pretty dense, so you can likely boil it.
Just a couple of thoughts. There are always alternatives that will work.
B