There is moisture behind the enclosure. my suggestion is remove and replace.

Ours is like that too. I was positive when we redid it in 2012 or 2013 that I'd gotten one that didn't have that seam aaaaand lo and behold, nope. Irritates the crap out of me. My dad's answer - make sure you dry it out. Um... hello? I got a set of these little nylon brushes and scrub the hell out of it now and again, but when I clean the enclosure and rinse it, chunks of mold wash out of the seams constantly. I've got no suggestions, but I totally feel you on it.

Remove and replace the entire tub?

Shouldn't the seam be caulked?

I would spray it with 3% hydrogen peroxide and let it sit at least 10 minutes. You might also saturate paper towels with hydrogen peroxide and let it sit overnight. To clean it out I'd use an old tooth brush or a small bottle brush.

We had that problem at our camp. There was nothing wrong with the tub or structure. It was poor air circulation especially in the summer. Bleach would kill the mold, but it wouldn't take long before it was back. I found the fix on the internet. Kill all the mold with bleach and clean the seam well , then after every shower spray the area with vinegar. The vinegar wouldn't remove existing mold, but the acid in the vinegar stops the mold from forming. I haven't had mold form in 6 months since I started doing it. I just keep a little spray bottle of vinegar by the tub.

@rrah the seam is caulked but the mold was growing on it and basically ate away a lot of it.

The mold may be caused by moisture in the wall. Everything you do on the outside will not affect the moisture inside the wall.

The tub is not the problem, there may be a leak in your enclosure.. This is text book water intrusion.

"This is text book water intrusion."

How so? This is a fiberglass surround and that is the seam where the pcs overlap. Moisture is just sitting in there are the pieces are not pitched for the water to run out. Water sits and mold sets in.