It always says this when you map Asteroid Rings for Hotspots.
So far, I've only ever seen "(0 Clusters)" Message going with it, never anything else.
I can only assume that something in the scanning/Info mechanics of probing Rings is using some internal element from Asteroid Belt Clusters.
And Hotspots aren't "it", a Ring with 30 Hotspots will still report as "0 Clusters" after mapping it.
IMHO it can be ignored similar to many placeholder/bugged strings you can get occasionally.