Did you happen to solve the problem? I am looking to do the same thing.

You'll want to use pipeline to fetch data and calculate factors. The 'data' method is deprecated.

To get the average of the previous 200 days close prices simply use the built in factor SimpleMovingAverage...

from quantopian.pipeline.factors import SimpleMovingAverage  
sma_200 = SimpleMovingAverage(inputs=[USEquityPricing.close], window_length=200)

There isn't a built in factor for getting yesterdays value. So, one would need to write a simple custom factor. Maybe something like this.

from quantopian.pipeline import CustomFactor  
import numpy as np

class SMA_200_Yesterday(CustomFactor):  
    inputs = [USEquityPricing.close]  
    # set the window length to 1 plus the moving average window  
    window_length = 201

    def compute(self, today, assets, out, close):  
    # Take the mean of previous days but exclude the latest date  
    out[:] = np.nanmean(close[0:-1], axis=0)


sma_200_yesterday = SMA_200_Yesterday()

With these two factors you could get the difference like this

sma_delta = sma_200 - sma_200_yesterday

This difference could also be calculated directly in a single custom factor. Perhaps like this..

from quantopian.pipeline import CustomFactor  
import numpy as np

class SMA_200_Yesterday_Diff(CustomFactor):  
    inputs = [USEquityPricing.close]  
    # set the window length to 1 plus the moving average window  
    window_length = 201

    def compute(self, today, assets, out, close):  
    # Take the mean of previous days but exclude the latest date  
    out[:] = np.nanmean(close[1:], axis=0) - np.nanmean(close[0:-1], axis=0)