liquid walking / water walking

I been wanting to make a potion that allows walking on water or walking on lava, my idea was to make a block appear underneath the player(a slab to be more specific) and once the player moves of the slab it vanishes, but ive had trouble with making a slab that appears on the upper part of the block or that dissapears after the player leaves, maybe if anyone has a code how to make ice but i could make it into nothing instead of it turning into water. Anyways ive been trying to implement this for like 1 hour or so and found nothing online maybe i will have more luck this way.