I know where you mean. Leave that screen and go back one, there should be a well and a ledge, go up via one of them & there will be a sleeping fleech, sneak past it and go to the next screen. Pull the lever, possess the Scrab and kill the Fleech. Now go to where the Fleech was and theres an Invisibility Ghost Lock, slap it & you're sorted!
I THINK thats right, if it's not, I'm really sorry. Hope I could help.