What is the precipitate in my used C-41 jug?

Hi. Long time reader first time poster.
I am using the Kodak C-41 kit (separate bleach and fixer not iron based blix) with replenishment. I have been dumping the used chemicals all together into a large 5L jug to one day take it to a proper facility.
I noticed that the jug has a white-gray flaky precipitate in the bottom. Is this metallic silver or is this something else? What else is solid that the C-41 chems mixed together could even produce?

Welcome aboard @kaarelp2rtel !
The sludge is probably mostly silver. However, there will also be some amount of oxidized developer as well as some other stuff. It's basically a crap shoot.

For proper disposal, it's best to keep the liquids separated so they can be disposed of optimally. This is assuming that the waste disposal facility will spend some attention to what's in the waste to begin with, which is not guaranteed (and arguably not even very likely).

There's another reason why it may not be best to dump everything together. For instance, the bleach can easily be regenerated/replenished and kept alive basically forever. Since this is the most expensive component of a C41 chemistry set, it makes sense to not dump it prematurely (or, indeed, at all). The fixer is of the most concern since it contains dissolved silver which has some influence on water systems if it were to be dumped down the drain. Desilvering the fixer is fairly easy using steel wool or tinfoil, leaving a fairly low-impact waste stream. The developer will oxidize readily, leaving a similarly fairly low-impact waste stream.

Overall, your waste disposal approach is well-intended, but probably not optimal for a typical home user.