Wintering Cymbidiums and what the heck is this?

[Nlamr]

Alright, first off I have to winter my cymbidiums indoors, in my house.... I have a very bright window sill that stays very cool, like 10-15 degrees cooler than the rest of the house because of the sheer curtain.... During the colder days it has stayed right in the mid to upper 50s, nights I don't know because I'm not up at 4-5 am when the nights get the coldest... So far they have faired well, but we have yet to hit the truely fridgid months.... So my question, how cold is too cold for a steady temperature? And what signs will I see if they are in fact getting too cold? Also I have been waiting over a month now, is this a new growth or a spike? It is growing painfully slow..... Could this be due to temperature?