Nems are some of the more challenging animals to keep in this hobby, requiring pristine water conditions and stability. Two traits that not often found in new tanks. They also require higher than average lighting, higher than corals requires. We dont recommend that nems be kept in tanks younger than a year because of this.
What are your water parameters? Any amounts of nitrates are intolerable, as are phosphates.
Triggerhappy - I do hope you are giving some extra TLC to the white nem you have, especially if that is a H. magnificia, because they are very hard to keep and that one is looking really sick.
And what you called a seabee nem looks more like a hairy mushroom to me, but its for sure not a seabee nem.