I agree....keep an eye on it. If you see no ill effects to your pond, I wouldn't fret over it too much. When my pond was renovated in 2014, I had a reddish brown line of oily seepage coming out of a seam about 8' down in one corner of the pond...not much, just enough to notice. When the pond was compacted and filled, I have never seen any residue on the surface what so ever. An old neighbor that's lived there all his life (and has several ponds) indicated that he's seen the same phenomenon over the years and never knew what it was....just something leaching out a seam in the strata of the soil.

...when in doubt...set the hook...