I had a cup of hot chocolate maybe a few months ago and completely forgot about it and its grown a thick layer of white/greyish mold can i clean and reuse it or should i just throw it out?
Assuming that the cup is made from ceramic or glass there will not be any problem in cleaning and reusing it.
There may be an issue with how things react with metal over time but I sincerely doubt it.
Clean it with soapy water and rinse with very hot water.
Possibly in addition to, fill with hot water, add 1 teaspoon bleach, stir briefly, let sit 1-2 hours, then rinse well. The bleach will kill anything in there for sure.
Keep in mind that the body does not get sick because there is a single bacterium or single mold spore ingested. The body gets sick when there are more mold spores that overrun a person's immune system. Thus it's to your advantage to eat foods that help the immune system, and gut bacteria, and avoid foods that hurt gut bacteria like processed carbs, processed sugars, artificial sweeteners.