Yes: according to Mills it's this de-excitation stage which releases the extra energy. According to his studies (example), it should be mainly in the form of UV/EUV radiation which can be easily converted to heat. Once Hydrinos are formed, they're not reused; they're supposed to be a very stable, difficult to utilize (due to the size) form of matter.
A large enough population of Hydrinos in the picometer size range might be able to undergo nuclear reactions at room temperature by tunneling, at least in the case of deuterium ("Deuterinos"?). If we are to assume that Holmlid's ultra-dense hydrogen is the same thing as Hydrinos, then a novel type of meson-producing nuclear reactions should also be possible with both hydrogen and deuterium.
Stable compounds formed by a Hydrinos and regular atoms might be possible too. Mills recently claimed that as well and supposedly showed one manifestation in a couple relatively recent videos (1 and 2).
I'm not a Hydrino expert/advocate by the way, the subject has just several points in common with the work of other scientists.