I have a birds nest fern growing on volcanic rock on kitchen window sill. A white growth has recently flourished and I'm guessing this might be a fungus. Any suggestions on how to treat to get rid of it will be appreciated.

Efflorescence is a white, chalky substance that appears on the surface of natural stone, clay brick, concrete pavers or any other porous stone material.This moisture is drawn to the surface of the stone where it evaporates, leaving the salts behind as a white haze. Sometimes it can be very hard to remove and you will need a strong acidic liquid. Try water and with a toothbrush first and if that doesn't work then go to the next step.
Add undiluted vinegar and apply it to your stone with the plastic toothbrush. Make sure you don't get any on your plants. See if it removes the white residue. You may have to use a mix of deluded muriatic acid mixed with water.

I agree, it could be leaching of minerals from the lava rock but, I would not use any of Luke's cures if I wanted to keep my plant alive.

The other possibility is a mold - you could try spraying it with a fungicide. Otherwise, a toothbrush and some scrubbing is all I would recommend. Personally, after I determined it was mineral leaching and not mold, I would let nature take its course.

I like that mount. I have used crushed lava rock to pot up lithophytes before but never thought of using a block as a mount. These large pieces of lava rock used to be popular with Anthuriums mounted on them (the Andreanum types)

The white is minerals leached out of the rock. Unsightly but not harmful. Water and a toothbrush might take some off, and watering by dumping the whole plant in a bucket for a moment might (it depends on the rock) keep it from growing more.

Don't use vinegar (or any other acid) next to your fern. You'll kill it.

Thanks for all the helpful observations and suggestions.

I've cleaned it gently with a toothbrush and water and it's now looking good again.