Who will become queen mother if William ascends to the throne?

There won't be one. "Queen Mother" is not an actual position, it's a term used to refer to a Dowager Queen who is alsothe mother of the current reigning monarch.

This usage helps clarify exactly who's being spoken of. George VI's wife's name was Elizabeth, and while he was king, she was Queen Elizabeth.

George VI's eldest child is also named Elizabeth. When he died, she became Queen Elizabeth (Elizabeth II). Her mother, who had formerly been Queen Elizabeth, was then referred to as Queen Elizabeth the Queen Mother, allowing her to keep the courtesy style of "Queen" but making it clear she wasn't the Queen Regnant of the same name. (It had less of an issue for her when she became Queen (Consort); her mother-in-law was Queen Mary, so there was little likelihood of confusion.)

Here's one plausible scenario:

Elizabeth II kicks the bucket, and Charles assumes the throne. Then eventually George VII (as a regnal name, "Charles" has ... issues; his other names, Philip and Arthur, are strongly associated with France and legend respectively, so they're not likely either) dies as well. His widow won't be entitled to rule as Queen Regnant, so William becomes (likely) William V.

Even if Camilla does take the title "Queen" when Charles becomes King (not especially likely; it's anticipated that she will be styled as "Princess Consort") so that if she survives her husband she will be a Dowager Queen, she's not William's mother and won't be "Queen Mother." Also, there's not much likelihood of confusing Queen Camilla (again, she probably will never use that title) with Queen Catherine (the current Duchess of Cambridge).