Right off the bat I would start by checking my water parameters to see
if there is anything obvious. (I.e. ammonia, nitrites, nitrates and pH
to start) His lethargy started after you moved him the first time so
there may have been some acclimation issues. It returned only after you
moved him again. There are stressors associated with changing tanks so
it could be from the moves.

Can you describe the white stuff you're seeing? Is it coming out of him
as a result of feeding or is it hanging off of another part of the
body? White stringy feces may be a sign of internal parasites. Jungle's
Internal Parasite Guard or Parasite clear contain some pretty effective
ingredients for treating flagellate parasites.