That looks normal. I've always had hot water do that going into the "rinse" tank.
Here are all the possible causes:
A bad weep check valve could be letting high pressure soap back into the water lines. That would be the easiest to find and fix, just close the water line to the weep and run each bay one at a time and check the others for weep coming out. If running a bay causes the weep to run on the others, that bay has a bad weep check valve.
A low pressure chemical could be backing up into the tank. I have seen this happen many times, and it requires a bad pump inlet check valve AND a bad pump. The easiest way to check for this (assuming the spot free check valves are at the boom) is to run each bay on a low-pressure function (tire cleaner or presoak) for 30 seconds, don't need the trigger pulled, then run it on spot free until it runs clear, then run it on rinse. Nothing but water should come from the gun. If you get any significant amount of the low pressure soap, the pump is a problem AND the check valve to that pump is bad.
It's possible for soap to get pulled into the plumbing from a Hydrominder, but realistically that should not happen. Your water pressure would have to drop to negative for some reason, so in the middle of a busy day without seeing anything starve for water this isn't going to happen. There should be a hole in the discharge tube above the mixed chemical on each Hydrominder anyway, which will prevent this.
It could be salt from the softener, it's unlikely but I have seen it happen. Easy enough to test, just taste it and if it tastes salty, there you go.
Least likely, but I have seen this twice myself, is that soap is clinging to the insides of the high pressure lines. The first time I saw this, it started doing this on all the bays within a few months, probably because all the lines were the same age. The way we figured it out was that we noticed that going from soap to rinse you'd see a little suds in the rinse for as much as five minutes, but if you went from soap to high pressure wax, even for a couple seconds, then to rinse, the rinse was completely clear.