Are there any negative health indications from getting sea legs?
Key point -- 


  The sensation only occurred once on the ground and after completion of
  the XC.
  
  until I went to sleep (~11pm)


If you have no further symptoms or at least no more severe symptoms, then I see no reason to be too concerned about this, and certainly no reason to bring it up with an AME.  (But see added comment below re long-term career plans.)  Interestingly, after hours spent feeling certain repetitive sensations, whether due to to thermalling flight in a hang glider or sailplane, or bobbing up in down in a kayak, I have sometimes noticed a sense of the motion continuing later, but ONLY while laying down prior to falling asleep in the evening. These sensations may have been more related to the repetitive forces felt by the skeletal-musculature system (increased G-load, rhythmic variation in G-load), than to the vestibular-occular system, so perhaps they were fundamentally different from what you are describing.

Comment incorporated into answer--


  this is a medical condition called mal de debarquement syndrome or
  MdDS. Do a search on line, you will find literature on it. I developed
  a severe case after flying from Denmark to Los Angeles in a DC-7C in
  1960 which would flare up anytime I was in an enclosed space, like a
  clothes closet or stairwell. it did not subside until 10 years after
  that flight. – niels nielsen


A google search on that term produced other similar results of symptoms lasting many years-- remarkable.  Some rather interesting ideas may be found about just what is going on here, and how to treat it.  See for example https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5418223/


  We reasoned that if the notion of MdDS caused by maladaptation of the
  VOR holds true, a reversed VOR stimulus should mitigate or even cure
  the MdDS symptoms. Based on these findings, a treatment for MdDS was
  devised that consisted of rolling the head at the frequency of body
  rocking, while the subjects viewed a slowly moving visual surround.
  This postulate was tested in 24 MdDS patients employing cross-axis
  visual coupling (1). The results were promising: 75% of the MdDS
  patients had significant improvement in a 12-month observation, which
  was the first successful treatment of this debilitating illness. 


(Front Neurol. 2017; 8: 175.
Published online 2017 May 5. doi: 10.3389/fneur.2017.00175
PMCID: PMC5418223
PMID: 28529496
Treatment of the Mal de Debarquement Syndrome: A 1-Year Follow-up
Mingjia Dai,1 Bernard Cohen,1 Catherine Cho,2,3 Susan Shin,1 and Sergei B. Yakushin1,*)

It would appear that there are enormous gradations in how severe this sort of thing can be.  If you are simply feeling some sensations on the ground, after a flight, and they are gone the next morning, it would seem both un-necessary and unwise to mention them to an AME.

I would say that occasionally-recurring symptoms at the level you've reported might be reason to think twice about a career as a professional pilot-- lest the symptoms get worse-- but still not something that would suddenly and significantly compromise your ability to fly safely on any given instance, and therefore not something that you should feel compelled to report to an AME.  

Obviously I am speaking from the viewpoint of what is in your best interest as long as  it does not significantly conflict with safety.  An AME might well give a different opinion.