Why do these grey outlines appear around my transparent graphics when I export to PDF?

I am working on the cover for my employer's next seasonal newsletter, and I am running into a problem with some graphics. They are on a transparent background, and I suspect that has everything to do with it, but I don't know how to solve the issue and I am hoping someone can help. Disclaimer: I am not really a graphic designer, I am a library assistant who makes a seasonal newsletter for their employer (LOL), so I apologize if this is an common, easy to solve issue - I have not run into it before today.

I am tring to make the newsletter title font look like it's sitting in some freshly-fallen snow, so I have all these white graphics that look like small piles of snow, and I am using them to create a 3-D effect on a white background by layering them on top of my font, along the bottom edge. I also have some light blue graphics that are meant to look like ice, and they are set on top of the title font across the top, to look like hanging icicles.

It all looks great in Indesign, but when I export my document to PDF and open it in Acrobat, these grey outlines appear around all the transparent graphics (see screenshots). I have used graphics with transparent backgrounds for YEARS and I have never had this issue before today. I have had grey and black boxes appear around transparent graphics but never this grey outline.

The other caveat to this problem is that when I print the PDF file, the outlines are no longer there. They only appear on the screen.

We have the newsletter printed by a local printing company, and I am afraid to send it to them in this condition because I am unsure if these outlines will appear on the final product. They are not showing up when I print the document here, on our tiny little office printer, but what will show up on a 4-color offset printing press? Or is that not a factor and I am worrying about nothing?

Of course, we get to see a proof of the newsletter before they send it to press, so I would be able to see if the outlines are there before we print, but I don't want to waste the printer's time with having to solve this problem while they are waiting for me to approve the proof, and then have to send them a whole new PDF, effectively starting their whole pre-printing process over. If this is a problem I can solve now, a month before it is due at the printer, it will be a lot easier for all involved.

So, is there a common reason this happens? Is there a setting somewhere that is wrong? What can I do to stop this from happening? Sorry for all the question marks, but this has never happend in all the 10+ years I have been designing this newsletter and working with graphics that have transparent backgrounds.

Note about the graphics: The white "snow" graphics do not have any transparency settings on them within Indesign; they are opaque graphics that have transparent backgrounds. The light blue "ice" graphics are all set to 95% opacity to mimic the translucency of ice and are also on a transparent background. All of these graphics are PNG files.

Thank you to anyone who can help me!