Yes. When you hover the mouse over NodeXL, Analysis, Find Clusters in the Excel Ribbon, the explanatory text below will pop up. And yes, we plan additional algorithms in future releases, although we're not sure when.
Partition the graph into vertex clusters by looking at how the vertices are connected to one another.
The clusters are indicated by vertex color and shape when the workbook is read into the graph. All the vertices in one cluster might be red disks, for example.
The clusters get written to the Clusters and Cluster Vertices worksheets. You can also fill in these worksheets manually, but note that Find Clusters will clear any manually-entered clusters.
The Clusters checkbox in the Show/Hide ribbon group must be checked if you want the clusters to be read when the workbook is read into the graph.
Find Clusters uses the algorithm outlined in "Finding Community Structure in Mega-scale Social Networks," by Ken Wakita and Toshiyuki Tsurumi.