Pink slime in a bathroom is almost always Serratia marcescens, an airborne bacterium that is common in the environment and thrives on the phosphorus and fatty residues in soap, shampoo and body wash. It arrives through the air, not through the pipe, which is why no water filter you can buy will stop it and why cleaning is the actual answer.
Where the pink film comes from
The organism lives in dust, soil and moist environments generally, and it lands on wet surfaces continuously in any occupied house. It only becomes visible where it finds constant moisture and a food source, which is exactly what a shower corner, a toilet bowl waterline, a pet bowl or the grout under a dripping tap provides. Households often see it appear after a change in season, humidity or ventilation, not after any change in the water.
Why it is not a water quality problem
Chlorinated municipal water arrives with a disinfectant residual precisely to prevent growth in the distribution system, and that residual dissipates within minutes of the water sitting in an open bowl or on a tile. So the film forms downstream of everything treatment can reach. If you want to satisfy yourself the supply is fine, every community water system publishes an annual Consumer Confidence Report and the EPA maintains a finder for locating yours.
Getting rid of it and keeping it away
Scrub the affected surfaces rather than wiping them, because the film is protected by a biofilm layer that resists a casual pass with a cloth. Use a bathroom cleaner or a diluted bleach solution where the surface allows, and disinfect the toilet tank as well as the bowl, since the tank is often the reservoir that reseeds the waterline. Then attack the conditions: run the extractor fan longer, squeegee the shower, fix drips and let surfaces dry.
What not to buy for this
A shower filter, a whole house carbon filter, a softener or a reverse osmosis system will not stop pink film, because none of them treat the air or the soap residue the organism is eating. Softened water can even make it slightly more noticeable at first, since soap rinses differently. If you also have a genuine water complaint, taste, staining or a failed well test, treat that on its own evidence and keep the two problems separate.
Questions people ask about pink slime in bathroom
Is pink slime in the bathroom dangerous?
For most healthy people it is a nuisance rather than a hazard, but it should be cleaned away promptly and kept off surfaces that touch food or wounds.
Will a water filter stop pink residue?
No. The bacterium arrives through the air and feeds on soap residue, so filtering the incoming water changes nothing about where it grows.
Why did it start suddenly?
Usually a change in humidity, ventilation or cleaning routine, or a utility adjusting its disinfectant. Nothing needs to be wrong with your water.
How do I keep it from coming back?
Dry the surfaces. Run the fan longer, squeegee the shower, fix drips, and disinfect the toilet tank as well as the bowl on your regular clean.