There is nothing invalid in what you have showed. First of all, be aware that the brackets things are only part of Anki’s custom syntax. That is, [sound:...] is not valid HTML, but Anki will replace these with something valid when you review the card, to make it produce sound. Besides that, tags (things like <span ...> ... </span> where ... can be a lot of stuff) are perfectly valid HTML. In your case, <span style="color: var(--field-fg); font-family: -apple-system-body; background: var(--field-bg);"> ... </span> is a tag that applies some styling to the text inside (not very useful here since you have no text inside):
it will set the color of the text to the value of the variable --field-fg;
it will set the font the of text to -apple-system-body;
it will set the background color to the value of the variable --field-bg.
I don’t use Anki Mobile so I’m not sure if there is a better solution, but there should be a Fx button that strips a field from its formatting (which should basically remove any such tag). You can find more information in the manual.
Those tags are not intended to be added. I tried some basic editing/copy+pasting between fields, but could not cause them to appear for me. Could you please figure out the minimum set of steps that causes them to appear, and then provide the instructions to me so I can try to reproduce the problem?
Create two lines text which the line break Tag is ‘DIV’, then position the cursor at the beginning of LINE2, then delete with the back button, merge the two lines into one, then view the HTML, you will see that a bunch of garbage code is generated.