What do I do if I efiled this years return but it's still pending and I claimed not receiving any of my stimulus checks but the 600 dollar check just came in?

You cannot change information in your return once e-filed unless it is rejected for some reason. If your return is ultimately accepted, the IRS will reconcile the stimulus payments on your return as part of processing.    

So I can still go ahead and cash the check and they'll most likely just take it off of my return? Is that what you mean?

Yes, you can cash or deposit your check and spend it.  

The IRS is not accepting returns until February 12.  Returns that have been submitted are being held in Pending until the IRS opens its servers to receive them.  If your return is rejected, then you can edit it and correct the entry.  If it is accepted, the IRS will contact you about the error and let you know what action you might need to take.