What's the name of signal encoding used by WS2812 LEDs?
AFAIK it doesn't have a fixed name, apart from 'the WS2812 protocol'. As described, it is a fixed-cell pulse-width encoding. 

FYI: in practice, it isn't fixed-cell at all: provided that the pauses (T1L, T0L) are shorter than Treset, all that matters is the pulse width (T0H, T1H). This makes it a plain pulse-wdith encoding, and it makes bit-banging waaaaay easier.