yes, sure you can clear that table, yes that is a cache table it just stores the id of the users which should be shown for example on the second page of the search results, in order not to make a new query and request to the database which decreases the load on the server.
If the table grows, probably some users are staying online as this table, at least the search results cache of that user should be cleared out from the table, which means your cron is not properly configured