As for the motion, the words "traverse" or "translocate" can work well for an object moving within or following a surface. However, I don't think that all three of your criteria can be conveyed in a single word (i.e. 1. the mobility, 2. surface constraint, and 3. the object itself). You are describing a mobile/moving surface (object) constrained within another (greater) surface.
(surficial or coplanar)
"sector", "subsector", "section", or "subsection".
Possibly one of these words would suit the technical purpose you have in mind?
https://www.thefreedictionary.com/traverse
https://www.thefreedictionary.com/surficial
https://www.thefreedictionary.com/coplanar