ID this invert

NOTE: this is an ecosphere, not an aquarium

i made a saltwater ecosphere about 1-3 weeks ago. today i spotted this little critter on one of my plants ( the plant was wide and green (maybe lettuce algea?)) i also saw it on the other plant (long and yellow, possibly kelp?) and on my (storebought) live rocks. i am not sure if he came from the live rock or from the sand or ocean water. he is streamlind, mainly a whitish color with a single red line on his back. he is less then a centimeter long but big enough to see. i think he has little legs and some antennae-like structures near his front ( i think). i can't get a good picture or video where he doesn't appear as a moving blob. so despite the vague information can someone id this thing ( i think it may be a worm or shrimp).

 

google amphipod

I reckon thats what you will find it is

Steve

 

ya probubly a amphipod

 

possibly, although the pictures don't match to well with what he looks like, its kind of close.