There was a white mass found on an x-ray under one of my molars and my dentist referred to it as a calcified mass- any ideas what that is?

Sounds like: A benign condition we see often called condensing osteitis. Seen mostly around non vital teeth and is the result of infection from a bacteria of low virulence resulting in bone production rather than destruction. Seen mainly around molars and premolars. No worries.

Calcification: Normally this is a calcification of the bone surrounding the tooth root and is benign. Some outgrowths of the bone- called tori- most commonly appear in the middle of the palate on the upper or the inside right, left or both on mandible towards the tongue. If there is doubt, get a second opinion.

Not a problem: It's dark areas, missing bone, that indicates a problem. Dense or thick bone appears white and for the most part is a normal anatomic variation.

Mass: We spend a lot of time learning about the varying shades of black, white &gray. Some is is normal , some is pathologic- condensing osteitis- which isnt found under a healthy/unrestored tooth.( you didnt indicate) There is also a whole bunch of pathology which will appear white. If your dentist was worried, likely they refer to an Oral Surgeon. or you can ask for a referral if worried.

Just What He Said: Like your dentist said, it's probably just a "calcified mass." These are common throughout the body and aren't a problem. It's similar to having freckles and blemishes on your skin...they're not "normal," but they're so common, that everyone has some. If your dentist is sure that's it's just a benign calcified mass, then don't worry about it. It doesn't need treatment and it won't be a problem.