Radiator help......

this one is for those who are good with diagnosing issues with little info..lol

I’m working through an over heating issue, and we’ve address what we think it part of the problem. The other part is pictured here. Looking into the top of the radiator down the fill tube, you can see all these white granular things. At first I thought they were small stones, but after flushing the radiator, realized they were not. I still don’t know what it is, but it’s granular, white, and will separate when rolled or pinched in between your fingers - best description I have is it looked like wet dishwashing detergent. We are guessing it’s cutting down 10-15% of the radiators flow.

Does anyone know what this is, and even after the motor break That was done before motor install, could this still have come from the motor? - or other? Thanks in advance...from stumped...lol

346F7AAB-2AC5-44FF-A333-B7922EB875FA.jpeg