Night time critters question

Last night after my lights were out & obviously the light in the nook where the 110 gl is located, we realized we have a lot of slugs (without the shell) crawling around in the tank (at 12 or more in varying sizes). I'm guessing they are good to have and probably came from in the live rock.

My husband also noticed there are bugs that look something like a spider on various rocks & even on the tank glass. It looked as though their back of their bodies curved upward. They were sort of white in color and would hide when shining a flashlight on them. They didn't swim either just quickly crawled away. What are these possibly?

 

+1 stomatella, I'm guessing you'd see they are actually wearing a beret .

your "spiders" are a little more puzzling, sounds almost like micro/mini brittle stars. I've got them from 1/2 dollar size down to pencil eraser diameter.

 

We do have brittle stars in the tank and figured out those a long time ago.

I googled amphipods images and they look like those pics.