if you're struggling in the neutral, go back to marth's basic aggressive movement without committing to anything, although use more WD back out of your DD than you normally would, this will let you effectively DD camp while facing him as much as possible, which is stronger for reaction-based play than usual in this match. you don't want to face away from falcon like you would against other characters because his ground speed is so much better than yours so the ability to protect your back is more important than the raw projected amount of stage you can cover with your own ground control. the reason for facing forward is to both protect yourself and that you want to be able to stuff out his nair pokes with your own fair to grab.
the ability to wait for a nair and to attack his attack with superior range and priority is going to shut out one of his main two approaches from neutral in this match, the other being grab. it's pretty important that you wait and react with your fair to grab on his nair because he can grab you when you land in fair if you don't react. there's a point where human error exists and i can understand getting faked out here so when you land in fair, you should l cancel and buffer it directly into a dash to make the grab whiff and then maybe you can DD grab his grab on your fair. layering your defenses from neutral will go a long way to weakening his approaches on you.
if the falcon player is trying to react to you with a grab, it's okay to overextend after him to cover his dash away on the DD for the same reasons he can do it back to you, except falcon's WD back coverage isn't nearly as good. if you go this way, when you run extra far to catch his dash away, do dtilt. the idea isn't to lead into an advantage, but merely to disrupt his movement well enough that you can go back to a reactionary stance. whether your dtilt hits or not, you should still focus on reaction and treat the situation like an extension of the neutral game. you should also absolutely get the IASA off of the dtilt to cover yourself defensively, once again layering defenses in neutral. what you cancel into for the IASA is your choice, but i'd recommend against jumping just to minimize the risk of trades because marth hates trades. if you're not sure what to do, WD back is a pretty-idiot proof bet but you're going to miss a lot of chances to react to falcon if you do that from the space you're forfeiting by moving back. you can try a shortened WD back out of the dtilt for a compromise but the spacing is a finesse thing and you're just going to have to practice and use your best judgment to retain your ability to work with anything the falcon player gives you.
this match is won in the neutral game, and IMO marth has the advantage however slightly because of fair on his moves (usually nair), a better WD back, and the ability to disrupt him better if you're both movement camping. keep in mind that his punishment options are better than yours and that any time he can upair all day you're basically going to lose.