Should I worry about this? (does it reduce the effectiveness of the shingle). If yes, then question 2 come into play.
There's certainly more important things to worry about in life, but, yes, Moss will, over time, shorten the lifespan of asphalt shingles.
Can it be corrected in the area that are truly unsightly with a chemical or something? I have kids so it would need to be safe. If yes, question 3 comes into play.
Yes. Actually, you want to correct it over the entirety of the roof. You can purchase spray-on as well as granule based moss killer. That is what you want.
If some cleaning can be done to fix, is this a DIY or does a service have to be contacted?
You can DIY or you can hire a service. If you live in a moss-friendly region (such as the Pacific North West of the USA) then you'll find all sorts of people that do pretty much just this. Either way, be sure to hire people that use very gentle cleaning methods. You don't want power washing or anything of that sort.
Once corrected, is there any preventative measures that can be done to avoid this?
The only real remedy is to not shingle a roof with asphalt shingles in moss-prone regions. That's why you see a lot of standing seam metal roofing in wet areas. Barring that, you just need to make the moss prevention an annual chore.
Some claim that zinc strips will work (as zinc will kill the moss) though, at least around here, you can find plenty of roofs with zinc strips that also still have plenty of moss on them, so I tend to think that's a bit of a scam solution.
It's probably time to replace the pantry roof. But first, you need to understand how so much water got there that plants started growing on it! Is the pantry roof a much lower slope than the house roof it's attached to? How's the flashing between the house roof and the pantry roof? Are you in a rainy climate? Could the slate roof be leaking water onto wood framing or decking beneath the slate? What are the plants consuming for nutrients? Most things don't grow with only water... are they rooting in some wood?
Best Answer
My answer would be no you cannot. It may depend on your location, maybe not, but a metal roof will allow condensation to form on the underside and that will allow other things to grow like mildew, from the existing growth you have, or keep the other alive too. You will need to clear the old roofing material and lay a condensation barrier down to keep the condensation from the metal from damaging the under structure of the roof.