Filter floss- reusable or throw away?

You could clean it to get more use out of it. Personally I throw it away because it's so cheap. I have used dirty filter floss to colonize new tanks with bacteria.

I clean mine under a strong jet of water - usually use my Python hose with my thumb over the end - and most of the dirt comes out. When it gets really dirty after a few rinses I replace it. I use floss that comes in a gigantic roll, I used to use the fluffy stuff that looks like a bag of cotton, but I find the kind you can cut into squares lasts longer and works better. Never used to bother rinsing the fluffy stuff, just threw it out.