What are nonstandard forms of written words called?
异形词 is the broad term.

A more specific, albeit obscure and probably not widely accepted term would be: 俗体词.

Usage of 俗体词 can be found on this Zhihu question: 英译中里俗体词和异音词如何处理？ 


  俗体词（比如错音错意、用词不当等）


Although it is talking about English, it should be clear enough to be understood in Chinese as well.