Not sure what's going on. It's growing endlessly, but the new leaves are frail and thin, not so uniformly 'heart-shaped', and very very pale in colour. No mottling with that telltale turtle pattern at all. I have it, originally, on the outskirts of my growlight setup, but the vines grow towards the light and are currently inches from the bulb. It grew like this in my north-facing window, as well. When I water, the lower leaves do plump up, but these newer leaves never do, so I don't believe it is a case of underwatering.

If the end leaves are thin and frail, its not getting enough water (only enough to plump up the leaves closest to the roots). If the leaves are losing their color, not enough light.

Do you water until some comes out the bottom? Do you check to see if the soil is getting wet when you water (by either using a skewer or by weight of the pot)? They take a little more water than a regular succulent - I water as soon as the soil is dry.

My plant is variegated. The top photo are leaves growing under grow lights and in a window. The bottom photo are the leaves that have grown below the edge of the window and out of the light.

Thanks for your reply :)
Yes, I bottom-water actually when the pot gets quite light, and I actually rely on the lower leaves (wait till they're bendy). It's /directly/ under my grow lights and each new set of leaves comes out like this. When they're teeny tiny, they look as though they may have the veination. Then they mature and it's pale and sickly. I'll try to water more frequently, though, I've been holding back since when I first heard of them, I got the notion that they'd fall apart at the sight of water much like lithops haha