Whoa - makes me want to see what gunk/corrosion i have going on in there. If it scraped off and the metal is intact - may be a good sign. It looks like some sort of leechable organic residue, maybe from plactics in the loop? but thats alot of stuff collecting there.

Seems to be some sediments. It is strange that you can scratch it away and see the nickel surface again.

Where were these sensor mounted (material)?

Very unlikely but also possible: if the internal sensor is damaged and not covered by the foil anymore, there could be an electric current floating through this part which could cause problems like that.

You could try one of those white kitchen sponge looking things, there are generic ones like Wal*Mart "Miracle cleaning eraser", and I think there is a Mr. Clean version. They are not really abrasive but with some elbow grease they do a pretty fair job of cleaning without scratching. Just use some water or maybe a little dish soapy-water on the sponge.

Be aware that there are some types of metal corrosion that can actually be a benifit to the system by isolating some metals from the water. If the film is very difficult to remove and not flaking off, I would not worry about it unless it shows up inside the cooling blocks and is restricting flows
The first thing I would do is get some pH test strips from a fish tank or pool store and test the water inside your system. (I know, a little late this time, but maybe you can still find a little bit in the radiators?) A pure/distilled water system should be slightly basic, pH higher than 7, to minimize corrosion of metals.
It might be some form of Copper oxide which gets deposited to the nickle through some kind of electrolitic reaction.
Yes, corrosion (rust) depositis do affect temperature change response time, but the effect is not measurable or applicable in our application. Organic deposits, (soft sludge/mold/slime), on the other hand are very bad. They do restrict flow through small channels in cold plates and can block/reduce flow.
Bottom line: If you have to remove it with a razor blade, and the threads/wet surfaces are not pitted, don't worry too much about it. Just remember to look at it again in six months or so.