Feta cheese is a perishable good, with limited shelf life even when properly stored.

You failed to eat it within its normal window of consumption which should be about a week after the expiration date.

Yours was nearly four months passed.

Why should Trader Joe's give you a refund? That's entitlement.

Should I feel like I should get a refund when I find a moldy three-month old stick of celery in the bottom of my fridge's crisper bin?

NO.

That's a bummer nutcakes, but Trader Joe's is great about taking stuff back, even without a receipt, so I think you'll be okay.

Well presumably you know when you bought this and if march 18th would have been feasible as an expiration date. But I had the same reaction, that this had expired in March. While I see references to longer expiration dates for Feta in brine they sound more like on the order of a few months, not going on for a year.

Check date format of similar items in the dairy case this week.
If necessary, ask store personnel to interpret is the second # a date or a year.
If date, you misread label. Chalk it up to learning.
If year and you still have product, take it in for return.

Update: Trader Joe's took it back on a 'no questions asked' basis, although I showed the expiration date of Mar 2018 and told them the problem anyway. Customer service said it is possible that the container had a pinpoint puncture. Anyway, I should add that I have enjoyed this product many times along with the slightly mellower and creamer sheep's milk feta from Israel from TJ. I have no problem with buying it in the future.
As a matter for anyone else, I am jonzing on watermelon and feta. Just plain is fine. Squeeze of lime fine. Some black and or red pepper, fine. Some mint or cilantro, fine. A little chopped cucumber, fine. Have not yet graduated to onion or olive oil on it. Why?