I think that @acvill more or less gets it, but I wanted to add a direct usage in context that makes clear the meaning.
I found this as the top hit when I google "glycogen mobilization". It has a lot of information and it is not straightforward, but it makes clear that mobilization is the process of breaking down glycogen into glucose. For instance, one slide says:
Glycogen phosphorylase usespyridoxal phosphate (PLP) a derivative of pyridixine (vitamine B6) as a coenzyme. B6 is required for the mobilization of glucose from glycogen.
As far as I can tell, "glycogen mobilization" means the mobilization of glucose from glycogen, therefore it is a special case of glucose mobilization in which the source molecule is glycogen. For example:
Glucose is a negative regulator of liver phosphorylase. – Glucose is not mobilized when glucose is abundant.
Additionally, wikipedia says:
...debranching enzymes mobilize glucose reserves from glycogen deposits in the liver.
glycogen debranching enzymes function in glycogen breakdown and glucose mobilization. When phosphorylase has digested a glycogen branch down to four glucose residues, it will not remove further residues. Glycogen debranching enzymes assist phosphorylase, the primary enzyme involved in glycogen breakdown, in the mobilization of glycogen stores.
So glycogen mobilization == glucose mobilization, in the context of human biology. That is admittedly kind of confusing.