Ok so i just pulled my dcp-3000 pump from my rock curing bin. Its been running 24 months straight with no breaks.
I detected a little grey/black on the papertowel i used on the initial biofilm. After washing clean and wiping again with the towel in 2nd photo there is hardly anything "rubbing off" of the iron armature.
I believe Jebao has made leaps and bounds on their pumps and material. So many people use them without issue i dont think you have anything to worry about.
"If in doubt.....ICP to rule it out."
ATI ICP-OES Complete Saltwater Water Test Kit
https://a.co/d/74x9JSQ