Large White Flatworms?

[Beverly]

Every once in awhile, I see animals in two of my tanks on the glass that look like large flatworms, only they are white and semi-transparent. They have an obvious mouth at the top, with a protrubrance on either side of the mouth that propels them quickly along the glass. Have also seen one moving on one of my small H. whitei seahorses, which sorta freaked me, and I managed to remove the thing from the horse and kill it

As these actually flatworms, or something else?