Nearly everything that appears in the game can be renamed, so I decided to see if the clusters could be renamed. They can't.
I zoomed out all the way, then clicked on a cluster to select it. The name of cluster appeared in the information box at the bottom-left of the screen. When I put the cursor on the name, I got a tool tip that indicated I could click to rename the cluster. This is identical to how other things are renamed.
However, when I clicked, nothing happened. The game completely ignored the click.
If something can be done to have the game recognize the click, I think clusters could be renamed.