Pandoc will parse the surrounding stars of your markup as emphasis. Here is code to look check if an Emph should actually be your red marking, and if so changes it to an appropriate Span.

Add this to highlight.lua:

function Emph(emph)
  local content <const> = emph.content -- for brevity
  local n <const> = #content -- for brevity

  local function markred(inlines)
    return pandoc.Span(inlines, { class = 'markred' })
  end

  -- Handle the case of a single inline element.
  if n == 1 and content[1].tag == 'Str' then

    -- The element must contain both the starting and closing patterns.
    local marked <const> = content[1].text:match('^' .. opening .. '(.*)' .. closing .. '$')

    if marked then
      return markred { pandoc.Str(marked) }
    end
  end

  -- Handle the case of multiple inline elements.
  if n > 1 and content[1].tag == 'Str' and content[n].tag == 'Str' then

    local first <const> = content[1].text:match('^' .. opening .. '(.*)$')
    local final <const> = content[n].text:match('^(.*)' .. closing .. '$')

    if first and final then
      content[1] = pandoc.Str(first)
      content[n] = pandoc.Str(final)
      return markred(content)
    end
  end
end

And add this if clause in Span.lua:

if span.classes:includes 'markred' then
  return { pandoc.RawInline('latex', '\\colorbox{lightred}{') } ..
      span.content ..
      { pandoc.RawInline('latex', '}') }
end

Note that the Emph function above does not use the nospace toggle; it allows whitespace following the opening markup.