No pic, just description... Bug on live rock ?

[Solga]

I have only seen this ... bug ... twice on my live rock, since I got it a couple months ago. It is a clear-ish, white-ish bug type creature and it moves very quickly, compared to the other things in my tank. Fast enough that I could not even have time to turn my head before it disappeared again. Both times I seen it, was when I dropped a few sinking pellets into the tank to help feed the serpent star hitchhiker. The way it moved, reminded me of a small, approximately 3/16" cockroach, crawling on the rock.
Any ideas ?

 

[Crabs McJones]

Amphipod or isopod most likely

 

[neonreef3d]

probably and amphipod

 

[Solga]

Thanks for the replies ! I was actually kind of shocked when I seen this thing ... I'm used to seeing the tiny ones, but have never seen one this big before.