Is there a difference between the two terms form and sequence in martial arts?
If you look at the forms tag you'll notice it's description is
a sequence of movements traditionally used in the practice and performance of a martial art.
An important word here is 'traditionally'. A form is not only a method to teach and learn a particular technique, it has also been used to preserve and pass on a proven technique in a formal and systematic manner.
The word sequence is used in martial arts as well (just search this site), but then it refers to a set of movements, not necessarily being a form.
Note that the term most people use in martial arts is the Japanese word 'kata', the Chinese 'taolu' or the Korean 'hyeong' or 'poomse', depending on the origin of the art one practices.
A form or kata is a set sequence of moves, always performed the same way.
Obviously you can also have a sequence (or combination) of moves that are not a form/kata, however in my (limited) travels I have never heard the word
sequence used as a formal noun for anything in any style (I've only heard it used as an adjective).
However there are literally millions of variations in styles and arts out there, who's to say someone hasn't started using the term in a formal way, or possibly that the word used in one of the more esoteric styles doesn't translate to English literally as 'sequence'?