Identify these critters!

[cameron808]

Just finished cycling my tank. No fish, just live rock. Can anyone tell me what these critters are?
1st video is what looks like baby sand fleas running around on the live rock.
https://youtu.be/LhuzQj8rp6Q

2nd video is some odd green thing. Sorry, the quality is poor and I have only seen this thing once when the ammonia was off the charts.
https://youtu.be/qax4hzBIcvw

Thanks guys!

 

[Sugar Magnolia]

First vid is harmless amphipods. Second one is too blurry.

 

[Dkuhlmann]

Maybe a brittle star on the second one? I agree with Sugar on the first.