Disclaimer: I'm not a Graylog expert.
To me, it looks like the Graylog extractor needs to be fixed, not the Instant AP.
After some reading, Graylog appears to work with 'extractors' that map fields in the log file to fields that the system can work with like date, source IP, etc. You should have an extractor specific for Aruba Instant, which I could not find in 3 minutes searching.