Daisy,
Yikes! The Hylo has survived the 'leech' effectively for three years. Maybe it's nearing the end of its resistance; or maybe it just needs more water (see previous replies) -- so I'm tempted to roll the dice on a smarter re-potting to encourage root growth, and more frequent watering.
I think I'll also re-jig the medium into a two-level arrangement: maybe an inch and a half of pure, fresh cactus medium over a bed of an inch or so of sand and pebbles (those tiny ones like gravel that you can buy in pet stores for fish tanks). That way, I'll be able to water more frequently (so the gymno on top can drink all it likes without sucking the hylo dry), but the medium will drain quickly, so the roots won't be kept in damp medium.
It's an experiment, and we'll see; but I'm reluctant to give up on the plant just yet.
Thanks,
