H.O.
Whether she is seeing spirits or not, perhaps you should look at it that way. Ask about the "girl". What she looks like, does she do or say anything? If it is a "spirit" (and you believe in this type of thing, you can do some things to "repel" them.
This is a good thing to do with her..as she can help chase them away. The fact is they shouldn't be there if they are there, and she can be in charge of sending this one away if she wants to. She should first sweep from where she sees the girl to her door, and then sweep it all out to an outside door (this can be "air sweeping"). When she has done this, sprinkle a little salt on the door, and declare "this room is for "your granddaughters name", no other little kids sleep here without MY permission. (It will seem silly talking to walls, but you'd be amazed at what works..especially with 6yr olds :) Also declare "That spirits no longer reside here as it is not safe for them" or some such..make it clear and consise. If she asks questions about it..use the ever useful "Because I said so" that's why :) If you'd like a friendly approach, you can say something like "All spirits should ask permission of the living adults in the house before contacting your daughter. You must meet all of them..and if they start rattling things tell them to dry up and get out. :)