How can one ensure that water reaches the skin during ablution when using exfoliators and moisturizers for the feet, and is there a way to distinguish between creams and moisturizers that permit ablution in their presence?

A single ruling cannot be applied to all creams. Rather, the governing principle is that anything which forms a layer preventing water from reaching the skin affects the validity of purification (taharah). However, something that is merely a color, like henna, does not affect it, nor does something that has a layer but dissolves with water when washed. Imam An-Nawawi said: "If there is wax, dough, henna, or similar substances on any part of one's limbs, and it prevents water from reaching any part of the limb, then their purification is invalid, whether the amount is large or small. But if only the trace or color of henna remains on the hand or other parts, without its substance, or the trace of a fluid oil such that water touches and flows over the skin of the limb but does not remain, then their purification is valid."