I use a rice cooker like the one shown below to cook dried mung beans and red beans (Adzuki beans) as well as rice of course.  After boiling and cooking, the rice cooker switches to a stable hot temperature to store the contents for a second meal, I suppose the idea being that bacteria don't grow readily when it's too hot.  That seems to work fine for all of them, except that while the rice is still white in the morning and the red beans still red, the mung beans turn dark brown in the morning. In fact they start changing color within a few hours. It doesn't matter if I've made them fairly dry or quite wet.  I cook them on different days, they are not mixed. Just the dried product and an equal or larger amount of tap water.  Is it possible to understand why the mung beans turn so dark brown if they stay hot? I don't notice any big change in taste, but the color looks quite discouraging. Is there anything I can add that would prohibit the color change?     
Milk proteins will coagulate at particular temperatures and Phs. You wrote that you used 2 Tbs of lemon juice but you didn't say how much milk you added that to. If you used too much milk then the mixture will not be acidic enough. Follow a recipe.  You also wrote that you boiled the milk once. I don't know if it is a language barrier issue but it sounds like you boiled the milk in the past. The milk needs to be at near boiling temperatures when you add the acid. The combination of heat and acid will cause the milk proteins to unravel (denature) and tangle up with each other (coagulate) which will result in the curd you are looking for.  If the milk has been handled well, that is if you didn't leave it out overnight, then it has likely not spoiled and you can try again by either adding more acid or bringing to a boil as necessary. 