“It’s so satisfying when I take a picture and hear the little sound,” Ms. de la Rochefordière said.

There are over 1,000 mosaics in Paris, where the first pixelated alien invaded the Bastille neighborhood in 1996. The retro tiles have long rewarded attentive Parisians with interludes of color in the otherwise muted city, mostly on street corners but also under bridges and on out-of-the-way curbs. When the phone game was introduced by the artist in 2014, that once-private satisfaction became a communal sport.

For every casual flâneur like Ms. de la Rochefordière, there is a hunter. “I spend so much time on Google Street View that once I arrive, I have the impression that I’ve already been there,” said Stéphanie Aubert, 50, a journalist in Marseille who is ranked 102 out of more than 151,000 players.

The several thousand mosaics by Invader beyond Paris are a boon for players who travel often and have a knack for adventure. Completists must snorkel for underwater Invader installations in Mexico and attend live events like a taping of “The Late Show With Stephen Colbert,” where the artist left a mosaic in 2015.

The game has brought fans together — some closer than others. Denis Gettliffe-Perez, 50, and Mélanie Perez-Gettliffe, 48, met in 2017 at a Pink Panther mosaic in the 11th Arrondissement. “We weren’t looking for love and Invader offered it to us,” said Ms. Perez-Gettliffe. (They married in December 2018.)

During the pandemic, new mosaics have popped up. Invader visited Marseille in August, giving locals dozens of new octopi and other Mediterranean-themed mosaics to collect. Ms. Aubert thought the invasion was a little clichéd — all those sun-soaked bottles of pastis — but was impressed that the artist made it beyond the Vieux Port and into the working-class districts in the north.

“It was great to see his choices in a city that I know so well,” she said, noting that in other places, like Djerba or Hong Kong, his artworks double as her travel guide. “I’m a pretty bad tourist actually, but this gives me a purpose.”