I am a salaried employee. Worked one hour on Monday but worked a total of 40+ hours for the week. Is it legal for my employer to take 7 hours of PTO for the day I left early even though I still worked over 40 hours?

No, As a salaried employee you cannot be docked for a partial day. It has nothing to do with the total hours you worked. You can be docked if you miss a full day, but not for a partial day.