The excessive saliva is due to the impairment of swallowing. Most people ( including completely healthy people ) produce about a quart and one half of saliva a day.
In ALS once bulbar impairment occurs, it is not that more saliva is produced. The drooling and excess appearance of secretions is caused by the inabilty to manage it when swallowing is impaired.
If you are talking about phlegm that is yellow or green or has any foul odor. An infection that needs prompt medical attention may be present. And you should be seen by a doctor.
If the phlegm is clear and is just thick secretions some medications may be useful in managing as well, these too can be discussed with a physician though this problem is typically not as urgent medically.
My husband uses scopalimine patches it reduces the saliva problem but does not completely eliminate it.