Rice cooker problem: film appears after cooking

Hot water and detergent might be enough, but after serious mold growth, I'd use a disinfectant. The easiest way to do this would be to disinfect your cooker with bleach, which is very effective at killing mold on non-porous surfaces. After thoroughly washing and rinsing your cooker, make a solution of 10 parts water to 1 part bleach and allow it to soak in your cooker for a few minutes, then dump it out and allow the cooker to air dry.

Using Bleach on Mold (note that the caveat here is that bleach isn't effective for porous surfaces. Anything that you're cooking on should be an acceptable surface for using bleach on.)

Once you get the hang of it, rice is as easy as pasta. One thing you say in the question that may be central to the difficulty you are having is that your lid is "half-closed". For the majority of rice cooking methods, not only should you keep the lid tightly closed, but you shouldn't even open it to check the rice until it has cooked close to long enough that it might be done. Look at the accepted answer here: Rice gets burnt and watery. That answer explains it more thoroughly.