Magento CE 2.0 Index Management Always Processing

Indexer status is stored in indexer_state table so try to update the status manually using:

update magento.indexer_state set status='invalid' where status ='working'

You can fix this from the command line by reseting the index which will invalidate the state of the index and remove the processing flag from the index.

php bin/magento indexer:reset

Optionally you may also append the index name to the command to just just reset that index.