Here are a few candidate compounds we have available today (rounding up a few of my comments.
One of the most obvious is Chlorine Trifluoride (ClF3)](https://en.wikipedia.org/wiki/Chlorine_trifluoride). It has niche uses in semiconductor manufacturing and was proposed as a rocket oxidiser. One of the researchers working with it, John D. Clark describes ClF3 as follows:
It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively. It can be kept in some of the ordinary structural metals... because of the formation of a thin film of insoluble metal fluoride that protects the bulk of the metal... If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes. [emphasis mine]
in his book Ignition! An Informal History of Liquid Rocket Propellants (which is worth a read)*
So ClF3, in contact with almost any material, will start a fire; it's the oxidiser so there's no need for air. Even glass spontaneously combusts in contact with it. The exceptions are metal fluorides, which make it possible to contain by using them as tank liners. It has a boiling point below room temperature and a fairly high vapour pressure, so would be expected to be gaseous by the time it made contact. This is ideal if you want it to degrade the hull rather than anything more dramatic (though in sufficient quantities dramatic is possible, as described here and especially in the link about a spillage. Note that a major reaction product in many cases is HF, which is nasty enough by itself (corrosive and poisonous).
As I commented, we also have Oxyacids such as Chloric acid (HClO3) and Peroxymonosulfuric acid (H2SO5) which would be useful against organic materials such as composite hulls as well as some metals. Even Piranha solution (sulphuric acid and hydrogen peroxide, mixed just before use) would attack composites.
I suggest though that with laser weapons commonplace, mirrored hulls would be used to provide some limited last-ditch protection. Rather than metal these could be dielectric mirrors (usually oxide material) so ClF3 would be ideal. Burning off the mirror coating would be just what was needed to make the hull more vulnerable.
* John D Clarke was also a published SF author and Isaac Asimov's boss for a while; Asimov wrote the foreword to Ignition!