Saw this while doing a water change

[ReefWelder]

I noticed this while I was doing a water change today; as I was adding water back in, it moved some sand around and I saw these things. Anyone know what this is? Should I be concerned?

 

[Sharkbait19]

I have this too and have no idea. Seems to die down eventually. Some kind of worm I think.

 

[((FORDTECH))]

I have same things under sand and on bottoms of frag plugs that I place on sand. Following