I have done Waterworks a ton and have recently noticed that Glurrup could very well be Sylster. To support this theory I have a few points, First being that in the very beginning of the dungeon he almost tells you not to do the dungeon, Secondly they are both Angler fish, Third they both have spears and almost the same armor. My theory is that he gets larger and more pale when he is mad turning into Sylster.