It's lichen ("Like-In") and it's common with fukiens, but not always the best idea if it becomes too extensive. You may be spraying a lot or just watering a lot, but spraying achieves almost nothing and there are better ways to humidify. Also, your tree may not be getting enough sunlight all day so try and give it more. You can peel/scrape off the lichen if you want - usually it'll appear only in little bits, rather than one big one - or not, but keep an eye on it to see that it doesn't take over the whole trunk, or branches.

Hmmm - doesn't look at all like lichen to me. Pic is hard to see, but ck out any one of several "liverworts" (aka scale moss), or it could just be algae.

Al

Tapla's right - liverwort. I tend to distinguish things as being moss or lichen (which we have loads of here), and forget about the others... but it's not moss, in any case.

I have it on bits and pieces of the soil of my ficus benjamina. It's not quite as wide-spread as the above pic. Will more sun eradicate it or should I pick those pieces out?

Sun won't necessarily eliminate it. If you have it, you're keeping your soil too moist, the soil was inappropriate to begin with, or the soil has collapsed. Removing it will temporarily fix the problem.

Al

It looks like algae to me, they love sun and water.