It actually “occurs” (currently) that those items are not hosted on the SketchUp Extension Warehouse.
(The icons are downloaded from the EW as a web resource in the interface.)
And it is a grayed Ruby icon signifying (as a default) it is loaded as a Ruby SketchUp extension.
Note that there is an open request issue to allow extension developers to somehow provide the manager interface the ability to “find” the icon image when it is not hosted on the EW.
So as Dave said it doesn’t actually “mean” anything purposefully other than being generic icon placeholders.