Does protection from evil and good protect the player from a polymorphed fey monster?

For Polymorph, no, because you cannot have a fey beast. Unfortunately, the filters https://www.dndbeyond.com/monsters don't make it obvious.

This 4th level spell transforms a creature into a beast, which is mutually exclusive from a fey creature.

For True Polymorph a 9th level spell, yes, because you can polymorph into any creature of same or lower CR, including fey.

As you know, Protection from Evil and Good does protect against fey creatures.

Edit: monster types are mutually exclusive, a creature can only be one of 14 types, not multiple.
https://www.dndbeyond.com/sources/dnd/mm-2014/introduction#Type
There are also sub-types (tags) e.g. humanoid (elf) and whilst an elf has fey ancestry that does not make it a fey creature.
Apologies if I'm telling you how to suck eggs.

Just elaborating the response above: Polymorph says that: “The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.” As the creature type is listed among the statistics and is not listed as being retained (unlike its alignment) the original creature type is replaced by “Beast”.  Thus, Protection from Evil and Good no longer works against that creature. 

(By contrast, a Vampire that shapechanges into a bat would still be affected by PfE&G, as “its statistics, other than its size and speed, are unchanged”.)

Thanks very much for the answers.