That is rust or iron oxide from the inner surface of the steerer.   
Luckily, an anodized aluminum surface like your Technomic Deluxe stem is 
pretty tough.  Some vinegar and fine steel wool should remove it without 
any further damage to the anodized surface.   If yours was a regular 
Technomic, it probably wouldn't be anodized, and thus could be scratched by 
the steel wool.   To prevent problems like this in the future, it is 
important to grease the inside of the steerer before assembly.  (A regular, 
non-anodized stem would be even worse, as the aluminum-steel interface sets 
you up for galvanic corrosion.  In that case, the stem would be attacked 
preferentially, leading to large volumes of aluminum oxide, and possibly a 
stuck stem in bad cases.)

Cameron Murphy
San Marcos, CA

On Sunday, October 16, 2016 at 12:29:46 PM UTC-7, Lungimsam wrote:
> Came off my Sam. Exposed stem is clean as you can see.clean. Stem sunk 
> into steerer tube has some surface corrosion of some sort on the aluminum 
> Technomic deluxe them. How do I get the corrosion off the stem and restore 
> it to its original lustre?

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