But this invasion of slimy creatures are surprisingly cute. Their cheerful smiles and luminous bodies brighten up grubby derelict buildings, drip through broken windows and swarm over construction walls.
These murals are the creation of street artist Sill, whose comic-book style creations can be seen across Sheffield.
Sill began living in the city four years ago and started documenting the street art scene before embarking on his own graffiti career.
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The 22-year-old likes to keep a low profile, as some of his work is not strictly legal.
“I got sucked in by documenting stuff and then I decided I wanted to try it out for myself.
“The idea had been floating around for a while. I was inspired by comic book art and wanted to do something with a back story.
“Sill are invaders that have come to Sheffield from outer space and I’m documenting how they are living on earth. They’re mostly friendly but I have created a couple of villains.”
The aliens are a hit on social media and Sill says he has received positive feedback from fans.
“A lot of people actually think they’re cute. I enjoy hearing things like that and knowing that people like them, although I did enjoy drawing the darker stuff but people have said the happier ones have cheered them up. People have also said they look like wine gums.”
And the invasion is spreading – with the help of stickers Sill’s fans are spreading the aliens across the globe. They have been putting them up around the city and some have even put them up for him when they go on holiday.
“It is nice to stumble across ones that I haven’t put up and people send me pictures of them,” Sill says.
Despite the illegal nature of this work, the street artist said he has not had much bother from authorities. Sheffielders seem to embrace street art compared to other cities in the UK.
“I’ve never really run into any trouble. Sheffield seems to be quite lenient on leaving pieces up than elsewhere and there are lots of legal pieces that have been commissioned here too. I’ve never really had much backlash.”
With Sheffield’s walls bursting with colourful street art, Sill is just happy to be a part of it.
He says he highly rates Steel City-based artists like Phlegm and Coloquix.
“There’s so many great artists here who are just incredible, but I don’t have a favourite there’s so many different styles.
“The talent in Sheffield is overwhelming.
“ There’s is such a great community of people.”
At the moment Sill is painting for fun but says he wouldn’t rule out making a living from his work if he got the chance.
He says: “I don’t do much commercial stuff but that’s not to say I wouldn’t if something came up.
“I just really like painting.”
As Sill continues to take over the world future work will see the aliens’ evolution into ‘cyber-punk style robots with exposed wires all hooked up to machines’. So look out Sill will be taking over a wall near you soon.