How do I use beer barm to make sourdough?
What you've got there is a mix of precipitated proteins from the wort (the "trub") mixed with Saccharomyces cerevisiae spores, and also various compounds from the hops. It's on the bottom of the beer because you're making an ale; only lagers have the yeast floating on the top, and then only during active fermentation.

Taste a bit. Then drink something to get the awful taste out of your mouth. :-)

You can absolutely use those spores to start a sourdough starter. The hop compounds may slightly retard the growth of the bacteria which you want in a sourdough starter, but after a couple of feedings they'll be basically gone. If you're worried about that (or the taste) you can use a process called "yeast washing" to get most of the trub out of the yeast.

As for whether you should do this? I wouldn't bother. The ultra-pure strain of S. cerevisiae that's been selectively bred for beer fermentation is eventually going to be crowded out by some other, scrappier wild yeast. Honestly, the most successful sourdough cultures I've started have been made of flour, water, and kismet.

Historically, yeast cake from beer brewing has been used for baking bread directly (rather than through propagating a starter over time), because it was an easy way to get a large quantity of a pure strain of yeast. That is, it was used to make non-sourdough bread. Now we have packets of instant yeast, for beer and for bread. Since you're using the instant yeast for beer, I don't know why you wouldn't use (a different) instant yeast for bread.