That's the Find/Change dialog you've shown there, not spell check. I don't know any way to eliminat master pages from Spell check, but you can eliminate any text from the check by changing the language assigned to it to [No Language] and that might be enough to solve the master page problem.
Is there actually misspelled text on the master page?
There is no misspelling on the master pages. But I have heard of other people having a problem such as this. I will try your solution and see if that works.