I rebuilt a loop a few months ago and looked at the reservoir just now. There is some sort of substance that appears caught in there. It looks like the blobs in a lava lamp, except there isn't a specific color to the blobs, they're mostly just darker than the DP Ultra that they're swirling in. Admittedly, some of that may be due to my red-blue color blindness.

I don't think just is simply radiator flux since the loop itself isn't new. The radiators and pump are about 6 years old and the blocks are 3 years old. The only new item in the loop is the Aquacomputer EPDM tubing that replaced some old clear tubing. The coolant used was new pure DP Ultra Clear. Prior to rebuilding the loop, it had been running pure DP Ultra Clear for approximately 10mos.

Is this because I should be running a biocide? Or because of something else?

These are some of the blobs. They're pretty close to the density of the coolant but will sink if surface tension is broken.