# In place of `if abs(back[i,j]-mask[i,j])<5`
abs_diff = np.fabs(back - mask)
diff_lt5 = abs_diff < 5

# In place of `mask.itemset(i,j,0)` for the elements satisfying condition
zeroed_mask = mask.copy()
zeroed_mask[diff_lt5] = 0

# To find offsets not satisfying `mask.item(i,j)==mask.item(i,j+1)`
adj_diff = np.zeros_like(diff_lt5)
adj_diff[:,:-1] = (zeroed_mask[:,:-1] != mask[:,1:])

# Combine this with the previous condition (difference < 5)
final_mask = np.logical_and(diff_lt5, adj_diff)

# Get coordinates of all the pixels satisfying conditions...
coords = np.transpose(np.nonzero(final_mask))