Answer:
While performing any kind of experiment in lab one must be very careful about contamination from other resources.
If you had stored the beaker uncovered for a week, there are 100% chances that your sample will get contaminated either from airborne bacteria in the atmosphere of the lab, other bacteria's from surroundings, or viruses or fungi or any other kind of microbes. If beaker is open there also dust particles that penetrate in it.
So, It is necessary that you should cover your beaker properly with plastic or glass lid or aluminum foil etc while incubation for the smooth results of your experiment.