Hello! I just got a nice Macho Fern (which i believe is also called a Giant Boston Fern) from Home Depot. I noticed it was covered in brown dust (dust is actually rectangular particles) so it put it in the shower for a bit to wash it off. Its been about 3 days since the shower, and i'm seeing the brown dusting on the tops of the leaves again -- not sure if i just didn't do a good job with the showering, or if the dust is coming back.

I couldn't find anything online that talks to pests that would cause this brown/rusty dust on the TOPS of the leaves, but i'm stumped. Does anyone know what this is? Like i said, what's throwing me for a loop is that the dust particles look like little lines or rectangles. Any help would be greatly appreciated!