Critter ID

[Goodersgold]

Hello all, seen this in my tank for the first time today, is it a Nudibranch? no chance for a picture with whites as it has now disappeared into it’s tiny cave

 

[Saltyreef]

Hello all, seen this in my tank for the first time today, is it a Nudibranch? no chance for a picture with whites as it has now disappeared into it’s tiny cave
View attachment 3091101

Bristle worm

 

[bnord]

Most are harmless and good detritivores