Stuff in brake fluid?

It is obviously something deteriorating. Might be rusting metal tubing, might be hoses/ rubbers disintegrating, if it uses plastic pistons, they might be.....or just plain rust from the cast iron in the master and calipers. In other words, darn near anything inside there. I'd inspect, replace, rebuild anything that has not been in say, 5 years, flush out ALL fluid and replace, and then keep a wary eye on it.