Is pink shower curtain scum a good sign?

You read correctly! From Dr. Falkinham's paper:

(14) Pink Slime and NTM. Recently, by reading the article of Feazel et al. (2009) on
NTM in showerheads, we discovered that whenever a biofilm sample had salmon
pink-pigmented bacteria (Methylobacterium), there were no NTM. Based on that
information, we looked closely at the water in our Philadelphia M. avium patient
household data and confirmed that if Methylobacterium were present, Mycobacterium
were absent (Falkinham et al., 2016). Further work documented that a biofilm of
Methylobacterium spp. cells prevented the adherence of M. avium cells and a M. avium
biofilm prevented Methylobacterium spp. adherence (Munoz-Egea et al., 2017).
Evidently, the two cannot coexist in biofilms. Taps or showerheads in a home with
NTM don’t have Methylobacterium. As not all taps or showerheads in a home will have
NTM, use those that show evidence of a pink scum or film; the pink scum will be seen
on shower curtains, shower walls, or in crevasses in a shower or sink. As killed
Methylobacterium spp. cells can inhibit M. avium or M. abscessus adherence, I need to
initiate a project to identify the cellular Methylobacterium spp. component is sufficient
to inhibit M. avium adherence and produce that for premise plumbing and medical
equipment.

Here's the whole paper for your reference:

A follow-up question for you, @mholmm , can your pink slime be easily cleaned? I noticed a tinge of pink behind (old) tile and wonder if it's the same thing? It is not easily cleaned, however. Chlorine seems to eradicate it albeit very slowly.

I wash the shower curtain with detergent and borax added and it comes out. I don't find it hard to scrub stains with regular bathtub cleansers. Any stain behind silicon sealant around the tub and shower can be hard to get clean. I have pealed off old sealant and redone.

Well that is very interesting and certainly a good and wonderful thing to know. . I’ve never seen slime anywhere in any bathroom or faucet or shower in any house we have ever owned. Is it common to have slime?? And I have MAC and other lovely things. Maybe I clean too much. Irene5

Pink sline can be caused by other bacteria such as Serratia marcesens, a potential pathogen. Therefore one would need to know what is causing the film to know if beneficial against MAC.

I agree with Nicholsig about Serratia Marcescens being a cause of pink growing in moist areas of bathroom. It's an organisms that is usually, but not always, hospital acquired. It causes a pink film along catheter tubing and I'd thought it was the cause of any pink film in bathroom for many years. Learned something new. I checked for ways to tell them apart, but only found info describing lab ID differences.
I think if we find pink gunk, just glove, mask and clean any visible film. The biofilm is usually deeper in pipes, I think.

Methylobacterium residue doesn't look like slime (I used the word "scum" which wasn't the best word either). It gives a slight salmon-coloured tinge to the bottom of a shower curtain and sometimes a faint residue can be seen in hard to reach areas. It washes out easily and can be cleaned with the usual tub cleansers.

Serratia m. is a serious drug-resistant bacteria found in some hospitals (that is also pink) but that isn't what I was referring to.

So interesting. Thank you. Irene5

Wow, i’ve never been so happy to see the pink scum in my shower! This is great news!

I am always amazed at the new information that I gain from this site. I always wondered why when I clean my stainless steel sink in the kitchen and my bathroom sink and my shower every week that there is this pink/salmon colored film around edges of drains crevices. Now I know, and I'm happy I have it. Thank you for bringing up something that I didn't even know existed.