No - it's earnings that occurred after your dad passed.
Prudential owes you interest on the money they held for 6 years (if they had found you immediately 6 years ago there would have been no interest paid).
Think of it this way.... would you rather
1) get the life insurance proceeds and not the interest
2) get the life insurance proceeds and get the interest and pay taxes on the interest?
even if you have to pay taxes, I hope you'd agree #2 is a better deal.