Ok, it's not actually IN the Pond, more like living on top of the filter.
OK, so it may not be a real alien, at least not a live one. Looks more like
a bodily byproduct, you know ....

I was taking the camophlage off my filter, wooden flower pot, with a bucket
of flowers on top. When I got the wooden flower pot off the filter, there
was this 4" long grey speckled item, that looked a lot like a digestive
byproduct. Except that I can't see how anything large enough could get in
there to leave this mass. Could it be a slug or some kind of egg mass?