Here is the sample code how to compute MSE from the histogram fit model:
x = .... % Your data. You've got it.
% Handle of the figure is needed
HH = histfit(x,Nbins); % HISTOGRAM fit model build. You've got it
%% Separate out the data from HISTOGRAM Plot and Fit Model
Data = get(HH(1),'XData'); % HIST Data
FM = get(HH(2),'YData'); % FIT Model
MD = mean(DATA); % MEAN Of Bins
FM2 = mean(reshape(FM, 2, [])); % Mean Of Bin Edges Fit
ERR = MD - FM2; % ERR
SSE = sum(ERR.^2); % Sum-Squared Error
MSE = mean(ERR.^2); % Mean-Squared-Error
