Q.Hi two questions. My white parts of my peace lily are turning black and there’s white powder coming off of them. The leaves look
fin I don’t know what’s wrong also my Boston fern is turning brown and losing leaves

Certified GKH Gardening Expert
These both sound like fungal infections. This will require, mostly, preventative maintenance. Make sure to let the soil dry out down to about 2 inches deep before watering again. Indoor plants need time to dry out between watering. If they do not get a dry period then the result will be a fungal infection.
Treatment will be relatively straightforward. Next time that you water (and you should only need to do this once) just replace 1/4 of that water with peroxide. This will kill off the infection in the soil without harming your plant.
You will still expect to see much more damage before it gets better. It can be quite difficult to get rid of an infection once started, and treatment will do a slight amount of damage too. But there is still a chance to combat this.