From dgraph/posting/index.go at master · dgraph-io/dgraph · GitHub, I can see that this directory is a temporary directory that is used when a predicate is being reindexed. It should be automatically cleaned up after the reindexing process is done for a predicate. Seems like it wasn’t removed for you automatically for some reason.
If you don’t have any ongoing reindexing, you can delete the folder safely.