whats this fuzz growing in my new radiator?

[cas3]

some history first, new brassworks radiator, flushed when received. i have no running water in my shop, so i collect rain/snow in buckets, should be pure water. block was scraped, vacuumed, pres tested, found casting flaw and had it spray welded, which in the heating process (i assume) loosened up more crud, vacuumed, blew, scraped some more, should be the cleanest block in the western hemisphere. new motor was filled with rain water last year for first fire up, driven about 30 miles up and down my country road this fall, life ends in summer for me...work, now i am removing the radiator to drill a hole in the water pump for hot water heat, and i find this. i expected rust brown water, but this stuff seems more like a chemical reaction, or electrolysis, ...or somthin! the tubing is aluminum i got from the neighbors junk pile, he's a farmer, could be some old chemical residue? stuff probably was out side for decades before i got it. polished with mothers, antiseize applied to the pipes under the rubber. any ideas from the sharp guys here? i will clean it all up, and this should be the final time so it will get antifreeze next. just wonder what the heck. its late, i am going to bed, will check back in the morning...not 5 months from now!