As stated in @JCRM's answer the aquamarine beam in the video represents the field of view of MARA or MAscot RAdiometer.
But I think the funky hexagonal appearance is strange and counterintuitive, so I'll elaborate on what this funny-looking thing really is.
A radiometer measures intensity of radiation, and in this case it's thermal infrared radiation from Ryugu.
The reason that the video shows two color beams simultaneously is to indicate that it is important to know what it is that you are taking the temperature of. The FOV of the radiometer and the FOV of the camera overlap nicely when MASCOT is position as shown in the video. Focus information from the camera gives range, and that can provide analysis what it needs to know about the overlap if MASCOT is tilted funny, or is falling or tumbling in real time; this MASCOT rover tweet says that it was already on and recording even as MASCOT was slowly drifting toward the surface.
The paper A Mobile Asteroid Surface Scout (MASCOT) for the Hayabusa 2 Mission (GSFC, Researchgate, Space Science Reviews (paywall)) describes the radiometer:
MASCOT will measure the radiative flux emitted from the asteroid’s surface using thermopile sensors. Six individual filters will be employed to measure the flux
in the wavelength bands between 5.5-7, 8-9.5, 9.5-11.5, 13.5-15.5, 8-14, and 5-100 µm. The primary scientific goal of the radiometer is the determination of the asteroid’s thermal inertia, the secondary goal is the characterization of surface mineralogy.
To determine surface thermal inertia, the temperature of the asteroid’s surface will be measured over the period of a full rotation using wavelengths from 5 to 100 µm. The temperature sensors are capable of measuring the full temperature range expected on the
asteroid’s surface between 120 and 450 K.
In addition, the emissivity of the surface can be estimated from the flux in the band-pass filters from 5.5 to 7, 8 to 9.5, 9.5 to 11.5, 13.5 to 15.5, and 8 to 14 µm. Thermal inertia can then be determined from an investigation of the surface radiative energy balance.
If these filtered wavelength bands (especially the last one of 8 to 14 µm) look familliar, it's because it's roughly the same band and kind of filter that's integrated with the the little thermopile-based radiometer chip inside those IR thermometers they stick in your ear, that feed back to your cars' air conditioner control or that you point at the sky. The six bands correspond to the six-fold "honeycomb" object seen in the video.
When deployed, that has a cover over it with collimating apertures that define the FOV to a half-angle of 10 degrees as shown below, sitting immediately to the right of the exposed sensor head in the left half of the image.
above: From The MASCOT Radiometer MARA for the Hayabusa 2 Mission LCPM-10, Pasadena, June 18th- 20th, 2013.
However the shapes of the bandpasses of the filter and absorber combinations are not all nice and tidy. You can read more in Grot et al. The MASCOT Radiometer MARA for the Hayabusa 2 Mission EPSC Abstracts, Vol. 7 EPSC2012-50-2 2012, European Planetary Science Congress 2012
Source x2: mascot.cnes.fr (1st is cropped).
above: x2 From The MASCOT Radiometer MARA for the Hayabusa 2 Mission LCPM-10, Pasadena, June 18th- 20th, 2013.