Not authorized for query on

To solve this problem you have to:
1 - create a specific user in mongodb
connect into mongodb using mongo shell client with root password (mongo -u root -p root) Ex.:

root@7e1ef7680116:/# mongo -u root -p root
MongoDB shell version v4.0.13
connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("f554061b-bc27-4b7e-87a7-82768f021cd9") }
MongoDB server version: 4.0.13
>

After connect, enter into database (use mydb) and create an user (db.createUser({user: 'myuser',pwd: 'mypassword',roles: [ { role: 'readWrite', db: 'mydb' } ]})) Ex.:

> use mydb
> db.createUser({user: 'myuser',pwd: 'mypassword',roles: [ { role: 'readWrite', db: 'mydb' } ]})
Successfully added user: {
	"user" : "myuser",
	"roles" : [
		{
			"role" : "readWrite",
			"db" : "mydb"
		}
	]
}

2 - change mongodb configuration on elixir project
You have to connect into mongodb using url sintax with authSource params (mongodb://myuser:mypassword@mongodbhost:27017/mydb?authSource=mydb) Ex.:

defmodule ContentProxy.Application do
  @moduledoc false

  use Application
  import Supervisor.Spec

  def start(_type, _args) do
    children = [
      ContentProxyWeb.Endpoint,
      worker(Mongo, [[name: :mongo, url: "mongodb://myuser:mypassword@mongodbhost:27017/mydb?authSource=mydb", pool_size: 10]])
    ]

    {:ok, _} = Logger.add_backend(Sentry.LoggerBackend)

    opts = [strategy: :one_for_one, name: ContentProxy.Supervisor]
    Supervisor.start_link(children, opts)
  end

  def config_change(changed, _new, removed) do
    ContentProxyWeb.Endpoint.config_change(changed, removed)
    :ok
  end
end

I’ve tried the sintax worker(Mongo, [[name: :mongo, database: "mydb", seeds: ["myuser:mypassword@mongodb:27017"], pool_size: 10]]) but it didn’t work.

It looks like you have figured it out yourself, but you can also just use the :auth_source key instead of switching to the URL format.

thank you so much!