Are there grants and fellowships for PhD students past the first year, but before candidacy?
Yes. Several of the fellowship and scholarship mentioned in a previous question (Options other than the NSF for PhD level grants?) are open to non-first year graduate students.

For example, The EPA STAR Graduate Fellowship,  The Ford Foundation through the National Academies, NSA Fellowships, Google Research, and The Data Incubator all allow people to apply past the first year. 

The NSA, Google Research, and Data Incubators Fellowships all target computer science. "The Ford Foundation seeks to increase the diversity of the nation’s college and university faculties by increasing their ethnic and racial diversity, maximize the educational benefits of diversity, and increase the number of professors who can and will use diversity as a resource for enriching the education of all students." The EPA Fellowships target environmental science, which could include computational science applied to studying the environment. 

Edit note November 2019: The EPA no longer offers their fellowship program.