1: Organic Molecules
In chemistry, 'organic' refers to any compound containing carbon, excluding carbon dioxide. Since carbon has a million uses even to robots, such as forming the backbone of plastic, the robots should be familiar with organic compounds enough to tell the CUB how to deal with them.
Plastics and proteins are polymers made mostly of carbon. Considering the biggest difference is that proteins has a nitrogen atom for every 2 carbon atoms in the backbone, the CUBs' may lead it to conclude proteins are simply plastics with an unusually high number of trace elements. Therefor, it should clean collections of proteins (e.g. dead skin cells) they same way it would flecks of plastic: vacuum or brush it up.
Water also has a lot of uses for robots as a very strong but stable solvent. Since reactions almost always happen faster solution, water should be all over the place in chemical factories and laboratories. Again, the robots should have enough uses for water that they would bother programming the CUB should with instructions on what to do.
Sweat is a solution of water and a very few trace elements, so cleaning it should be the equivalent of a small chemical spill.
3: Human as a Whole Part 1
Even if the traces left by humans are nothing special, maybe the human itself is. So lets go step-by-step and see exactly what happens when a CUB climbs up a human the first time.
1: Move forward (CUB climbs up human)
2: Examine compounds on surface (CUB runs scans of the human's skin)
3: List detected compounds:
a) Dilute solution of ions in H2O (sweat)
b) Anomalous plastic (skin cells)
4: Reexamine plastic for anomalies
High levels of nitrogen detected
High levels of sodium detected
Abnormal carbon-hydrogen-oxygen groups detected
5: Consult safety protocols
No danger is posed to this this CUB by anomalous compounds. Continue cleaning protocol.
6: Engage cleaning mechanisms number 1 and 2
7: Apply cleaning mechanism 1 (however the CUB clean up minor spills)
8: Apply cleaning mechanism 2 (however the CUB clean up plastic)
9: Move forward...
This is how I interpret the CUB as you describe. The overall effect on a human would be insignificant.
You could change step 9 so it only moves forward if it no longer detects material to clean. In this case the CUB would just be stuck in a loop. Your body is always going to be producing sweat and skin is held together by a tough extracellular matrix that a simple cleaning is not going to damage. If the CUB did use tools harsh enough to damage human skin, then the CUB would also damage the plastic casings on regular robots. I suppose a CUB could get stuck in an infinite loop constantly using weaker techniques on the human until the human got hurt, but a good programmer would have told the CUB to move on if it cannot successfully clean a surface after a certain number of attempts. Besides, the human would simply manually remove the CUB long before it got to that point.
3: Humans as a Whole Part 2
I have been thinking of the CUB as very simple and basic, like a starfish or an automatic vacuum. Another way to think of it is that CUB are like the robots from Wall-E: a robot with advanced AI is given a goal and told to complete this goal however it sees fit. These types of robots have minds much closer to what we understand them as. While the primitive robot will try to clean whatever is in front of it, this more advanced robot will actually ask the question 'What is that' and try to identify new things by relating them to its prior experiences and knowledge. How would this type of CUB react to a human?
First, it would identify the things about a human that are different from the things it usually deals with. The human contains a large variety of organic compounds suspended liquid H2O, with a few other trace compounds randomly thrown about. This planet is located "deep in space". The closest star is about 4 light years away, so lets say the robot's planet is somewhere between 2 to 3 light years from the Sun. Is there anything at about that distance with a similar composition to a human...