What is causing dairy to become slimy?

The author's fiancée made Chè Thái, a fruit cocktail dessert, using an unusual liquid base of half-and-half and coffee creamer. After a few days, the dessert thickened and became slimy. The dessert was initially stored in a non-airtight Rubbermaid pitcher. A similar incident occurred with carrot juice stored in the same pitcher. The author suspects the pitcher's material or lack of airtight seal as the main culprit. Refrigerator temperature is considered a less likely cause due to past successful storage. The coffee creamer contains carrageenan, a thickening agent, which might have interacted poorly with the half-and-half. The remaining half-and-half and coffee creamer in their original containers remain liquid. The author seeks an explanation for the dessert's unusual thickening. The possibility of bacterial growth due to the non-airtight storage is also a strong contender.