Crypto-communist means being a secret communist. They keep themselves secret in order to infiltrate, to spy, to betray. NB the Wikipedia is ignoring its claim of being neutral in this case; something that tells us more about the Wiki than Crypto-communism.

Crypto-communism ex Wiki
Crypto-communism is a pejorative term implying a secret support for, or admiration of, communism. The term is used to imply that an individual or group keeps this support or admiration hidden to avoid political persecution or political suicide.
Naturally enough the Wiki does not give us examples of the guilty. Oppenheimer [ Jew ] gave cause for concern but the Wiki goes easy on him. Atomic spies include Morris Cohen [ Jew ], Klaus Fuchs, Harry Gold [ Jew ], David Greenglass [ Jew ], Theodore Hall [ Jew ], George Koval [ Jew ], Irving Lerner [?], Ethel and Julius Rosenberg [ Jews ], Irving Kaufman [ listed as a probable by VNN ], Saville Sax [ Jew ], Morton Sobell [ Jew ]. The Wikipedia's list of Atomic spies claims that most of these spies were American. It admits that they were very largely Jews in the indvidual entries. In fine the Wikipedia lies; by omission usually but lies none the less. The score is nine definite Jews out of twelve, a good indication of their treachery.