A Rough Introduction to Selective Crypsis

From Wikipedia:

In ecology, crypsis is the ability of an animal to avoid observation or detection by other animals. It may be a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean lifestyle, and mimicry. Crypsis can involve visual, olfactory (with pheromones), or auditory concealment. When it is visual, the term cryptic coloration, effectively a synonym for animal camouflage,^[a] is sometimes used, but many different methods of camouflage are employed by animals.

 Wikipedia defines Crypto-Judaism as

 the secret adherence to Judaism while publicly professing to be of another faith; practitioners are referred to as "crypto-Jews" (origin from Greek kryptos - κρυπτός, 'hidden'). The term crypto-Jew is also used to describe descendants of Jews who maintain some Jewish traditions of their ancestors while publicly adhering to other faiths.

This definition is limited, since it focuses on the traditions, faith, or religion of Judaism, without  mention of other ways Jewry can be expressed.

"Who is a Jew?" (Hebrew: מיהו יהודי‎‎ pronounced [ˈmihu jehuˈdi]) is a basic question about Jewish identity and considerations of Jewish self-identification. The question is based on ideas about Jewish personhood, which have cultural, ethnic religious, political, genealogical, and personal dimensions. 

Here, we see the first two modes of Selective Crypsis:
The first is the ability to shift back and forth between a Jew as a follower of a religion, and a Jew as  an ethnic category. Some state that the holding of both views simultaneously is impossible. Others state that both views should be held simultaneously. Others take one view over the other. Others still like to jump into either camp when most convenient.

The second is the ability for those Jews-by-Ethnicity, specifically Ashkenazim, to blend in with White or European people. Once again, we see some people that claim to be both Jewish and White, neither Jewish nor White (mostly Social Constructionists), Jewish to the exclusion of being White, White to the exclusion of being Jewish, and most troublingly, those that like to jump into either camp when most convenient.