It could be one or more of several other problems: 1. no trap in the floor drain 2. drian pipe too small or clogged with lint and detergent residue 3. the washer drain enters the main drain by a tee laying on its side that should be a wye instead 4. Floor drain trap has evaporated all its water out and no longer presents a water seal. 5. improper venting of either the washer drain or the floor drain.
Remedies: rent an electric snake (1/4 to 3/8") run it through the floor drain and go as far as you can, backing it out occasionally to clean lint off the end,
pour a quart of water in the floor drain; the reflection in the water in the drain should be a full circle.
If it is one of the other three problems, forget it you don't want to break up the slab. You could take the screen off the floor drain and install a plumber's test plug in the floor drain, but then when you have a flooded floor you will have to reach down in the drain to remove the plug.