Well, for a start, if the tree stump has still got bark on it, it will probably be housing all kinds of tiny mites and insects which over the next few days will migrate out into your living room. And bear in mind that over a period of time, the warm air indoors will gradually dry out the bark and it will eventually start peeling off in small pieces as it dries. If you just want to clean off all the dirt, you can try putting it in the bath (without water) or the base of your shower tray and spray it with warm water, leaving it there, or in a plastic bowl until all the water has drained off. That should remove surplus dirt, cobwebs etc and make it clean enough to sit on, especially if you put a cushion on top. However, if you place it on a carpet, it's possible that the natural juices and resins in the wood will leak out of the base of the log over a period of time and stain the flooring or carpet..