You see it behave as a particle when you cover a slide and it simply passes through the single slit.

“Particle” is the right word to describe the nature of a photon, and so is “wave.” When you collapse a wave function by “observing it” - basically the wave/particle interacting with something - you find it as a particle somewhere.

Eg, imagine an atom surrounded by electrons. Until the electrons are “observed” - interact with something - they exist in a probabilistic “electron cloud.” Depending on energy level and sub shell (ie, the “quantum numbers” you might have encountered in chemistry - remember 1s^2 2s^2 2p^6?) there are places that the electron is more likely to be. An electron in 1s is likely to be closer to the atoms nucleus when observed. But it could also be on the opposite side of the universe - small chance, but possible. It exists as a probabilistic wave though until that wavefunction is “collapsed.” (One of the silly things I like to point out when teaching is that conceivably all of your electrons could be out for a trip, and you could just phase through your chair)

This is why the photons going through the a single slit will behave like particles, and those through the double slit will behave like waves. They aren’t “collapsed” at the double slit. The single slit will only allow them to pass through as a particle.

But the vocab is unambiguous and correct - they are both “particles” and “waves.”