Transit of Mercury
Transit of Mercury
[Panel showing the sun partially obscured by clouds with faded gray blue sky around the sun, mainly upper right, as there are clouds in the lower left part. The image is very bright and blown out at the bottom left, obscuring most of the surface detail of the sun.  A small black dot is visible in the upper-right part of the sun's image.]
[Caption below panel, with a yellow unhappy smiley before the hash tag:] This photo of the transit of Mercury fried my telescope's imaging sensor 🙁 #NoFilter

This comic is in reference to the transit of the planet Mercury across the Sun on November 11, 2019 (the date of the publication of this comic), which appeared from Earth as a small black dot moving against the background of the Sun. Randall has made comics about solar transits before, albeit about the transit of the International Space Station, in 1828: ISS Solar Transit and 1830: ISS Solar Transit 2 . Viewing a solar transit requires a special lens filter to prevent the intense light from the Sun from burning out a telescope's imaging sensor.
The hashtag #nofilter is typically used on photo sharing sites, especially Instagram, to humblebrag about having encountered situations so photogenic that no further image enhancement (" filter ") is required to prepare them for general advertisement. In this comic, the hashtag is instead used to cap off an image about the predicament of the poster, where the lack of a proper astronomic filter has led to damage of personal property. The image shown on the comic is quite bright and blown out , and though the poster did manage to get a picture of Mercury, the sun's bright light permanently damaged their telescope.
The title text refers to a still different meaning of the word "filter"; it imagines a swimming pool growing green scum in the absence of a water filter , as opposed to a photographic or astronomic filter.
Although not directly referred to in this comic (although a variant was used in 1911: Defensive Profile , a third common variation of "No filter" is possibly alluded to here and can refer to someone who makes, or posts, tactlessly candid comments. While often this means comments that reflect the individual's actual views which are potentially offensive or socially unacceptable, it could also refer to someone who posts every mundane detail of their lives, such as what is growing in their swimming pool (as is shown in the title text). Multiple layers of meaning makes this pretty clever word play.
[Panel showing the sun partially obscured by clouds with faded gray blue sky around the sun, mainly upper right, as there are clouds in the lower left part. The image is very bright and blown out at the bottom left, obscuring most of the surface detail of the sun.  A small black dot is visible in the upper-right part of the sun's image.]
[Caption below panel, with a yellow unhappy smiley before the hash tag:] This photo of the transit of Mercury fried my telescope's imaging sensor 🙁 #NoFilter
