Apple/mystery snails make pink egg clumps which are above the water line. These clumps are about 1/2 - 1 inch in size, these are not apple/mystery snail eggs. MTS, have tiny live babies - they are TINY ice cream cone looking things. I have never seen the eggs of those super tiny ramshorns...and unfortunately I have lots of those so I think I would have noticed their eggs.
My 2 cents is that they are fish eggs. What was in the tank with the hornwort - rainbows? Hmm. Something which spawns on a mop... I'd hatch them and see. (But not in a tank I cared about!)
Cathy