Treat the area around it with wood preservative (containing a fungocide)if you have a wooden floor. Find out why the wood is damp enough to grow mushrooms and fix that. If the wood has been weakened you may have to replace floorboards or part thereof. The main thing is to prevent it spreading. It could also be in the floor joists which is more serious. It does need seeing too urgently.
In that case, Smurf, I'd want to check that the sub-floor ventilation is clear. It's not a big deal to have a bit of fungus above the floor. Under the floor might be more of a concern.
It does suggest water from somewhere. Quite possibly a leaky wastepipe. Worth investigating. You could start getting a bit of "wet rot". If the fruit is a nice pretty shade of pale yellow, then I'd be looking for the much more serious "dry rot"
I agree with the fungicide treatment though. A good dowsing.