Worm ID please

I have recently discovered a worm in my tank.

It is brownish and smoothe, no bristles, legs, nothing really. I do not even think it has a head.

I spot it with the red light, and when I turn on the normal light, it is almost like it "rolls" itself inside its own body.

It is hard to explain but I have seen it about 4 inches long, then the end seems to tuck itself inside and shrink all the way into the rock.

I wish I could get a pic or describe it better, but it is VERY sensitive to light.

Any thoughts??
 
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