Help to ID a flatworm like creature, sorry no picture!

[Snoopdog]

I can describe this thing very well, I got a good view of it with my lights out. I could not get a picture because he went inside of a rock before I could take one. It was maybe a bit smaller than the diameter of a dime. It was stop-sign shaped, almost like an umbrella shape. It was that tan translucent color, almost like the color of planaria. It was paper thin, same thickness of a flatworm. It did seem to have a central mouth in the middle. I noticed when it was moving that the edge would come off the rock and flap in the current, much like a flatworm moves. Before I kill it, does this sound like a problem flatworm? I am guessing my 6-line thinks it is too big to eat.

 

[JGT]

I can describe this thing very well, I got a good view of it with my lights out. I could not get a picture because he went inside of a rock before I could take one. It was maybe a bit smaller than the diameter of a dime. It was stop-sign shaped, almost like an umbrella shape. It was that tan translucent color, almost like the color of planaria. It was paper thin, same thickness of a flatworm. It did seem to have a central mouth in the middle. I noticed when it was moving that the edge would come off the rock and flap in the current, much like a flatworm moves. Before I kill it, does this sound like a problem flatworm? I am guessing my 6-line thinks it is too big to eat.

Could be, but without pic hard to say.

 

[Snoopdog]

Could be, but without pic hard to say.

Yeah I know. I looked tonight when the lights were out but could not find it.