what is the brown foam and sludge in my water softener brine tank?

"Iron Reducing Bacteria" create energy by oxidizing soluble iron in the water. In the process this creates insoluble iron which settles out and leaves iron staining. The bacteria themselves create biofilms or foamy messes in several areas, most notably toilet tanks and water heaters. Look in your toilet tank, if you have similar foam and iron staining you probably have iron reducing bacteria in the water.

If you do have iron reducing bacteria you have to deal with them at the source, your well. Generally this involves pouring bleach or another sanitizer down the well, pulling and circulating that through the plumbing system and then letting it stand in the pipes and well for 12-24 hours. This is not a permanent fix, but it can get you a few months respite. Any well company can assist with this process or you can find information online.

The problem is your water softener resin beads will be destroyed by this process, normally the water softener is bypassed during sanitization. You need to sanitize the plumbing system separately, and clean the water softener by hand. I recommend replacing the resin beads and sanitizing the resin tanks and brine tank at the same time.

A more expensive but permanent solution is to install water treatment systems to deal with the bacteria. Usually this involves chlorine injection, some contact time in a tank to kill the bacteria, and then running the water through a filter or reverse osmosis system to remove the dead bacteria and most of the chlorine. This is very expensive.

The first and easiest approach is to try an iron-specific additive in the water softener - there is both "water softener salt with additive mixed in" and products meant to clean the resin of iron such as Rescare or Crystal Clean. Here is an example of the salt version.

If it works, great, if not, on to more drastic measures, typically an iron removal filter before the water softener. For low concentrations of iron, a softener can be adequate, for higher concentrations it may not.

Iron bacteria can be fiendishly hard to actually kill off, as they may be present in the groundwater that your well is drawing from, so killing them off in the well may be only a temporary solution. Fortunately they are not harmful, merely annoying.

I see this as the result of not using the better quality salt for the brine tank as well as letting the salt reserve get down way too low, so that you are seeing things which are more often hidden!

Consider what salt is used and if there are no other signs of iron problems like a reddish color in the bathroom stool, etc. ignore the film and color as something that is normal in many cases. When the water level is so high that it is not hidden by the salt waiting to dissolve, you have very little reserve brine for the next regeneration cycle.

I like to keep mine topped off by adding more each time and doing it less often. Only the correct amount of salt will be dissolved into the water.