I've had little experience with mangoes but it looks like yours is sick. Please take a look:

https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-46.pdf

Tree is not lost :-) Lichen are harmless - ignore. A singe brown/dry leaf tip is not a strong indication of any fungal issues nor are a few aborted baby fruits. Investigate what is inside of the webbing, if anything.

Overall, the tree looks pretty good and I wouldn't rush to any chemical pesticides until I had confirmed there were issues.

It's not "lichen" -- it's powdery mildew, Oidium mangiferae Berthet, specific to mangoes and pretty serious. Requires spraying.

There is very clear evidence of lichen growth on stems and trunk. And I see NO signs of powdery mildew on the OP's plant that looks anything like the link you provided.

But then I am only a consulting horticulturist with a specialization in plant pathology......so what do I know :-)

Diagnosing a mango is tricky business:). What would I be looking for "inside the webbing"? There are some other places on the plant with thin strands between leaves that actually look like spider webs. But it's distinct from the thick white web associated with brown fruit buds (included in the photos).

First, powdery mildew seldom produces anything like a web. It is a surface pathogen and typically presents as a white or grayish film or powder on leaf surfaces. I don't see any of that.

The webbing could conceal an insect. Does it appear anywhere else besides those couple of attachments to the fruits?

This little black bug stops crawling whenever I want to take a photo. I think I just caught him casually on the plant, and he doesn't seem associated with the webs.

Here's a wide shot of one (of a few) spots with these wispy spider-like webs between leaves and panicles. You can just barely them out if you look close. See next series of photos for close-ups on a leaf with thicker web...

This is similar to the thicker web seen on the budding fruit and pannicles. I thought I saw something pinkish beneath the web at the bottom and tried to hold the leaf still and zoom in with my camera...

You can make out the fleshy beige discoloration beneath the bottom part of the web:

Unfortunately, this was the last shot before the little critter ran out from underneath the web and escaped (to another leaf?). It looked like a beige pale pink, tiny, maybe-spider? or mite? I couldn't count the legs exactly. But you were right, definitely an insect in the webs, though I didn't catch any in the ones on the buds.

Here's the later stage effect on the buds again:

So what are these guys? And what to do??? The other half of the plant looks so beautiful. Also, given your expertise, gardengal48, if you can recommend any academic resources to learn about this sort of entomology (or pests), I'd be very interested. Thanks!

A few more of a different part of the web, which looks like there might actually be a tiny spider in the middle, I can't tell it's so small? I only noticed it zooming in to the dead center of the photo. It looks different (darker) than the pale one that ran out of the web on the leaf (above):

another view, possible spider at top center of web? You can definitely see it if you zoom in (more than houzz will allow unfortunately):

different web entirely, but another black-like spider (?), top-center-right (zoom in):

No clue what's going on....

Webbing sure looks like a hiding place/protective covering for some creature.....so not a disease :-) And looking at local resources are probably best to see what sort of insect issues may be common to your area. Some will be more or less universal - like aphids or spider mites - but others will be very specific to location and/or plant species.

Leaf close ups definitely show bug encampments - get a magnifier and look at them with those.

It looked like a beige pale pink, tiny, maybe-spider? or mite?

Sounds like a predator mite. These don't suck plant juices.

do go out at night and see if you find anything ...never hurts to check

you might also contact your FL county Ag or extension office ... for local bug experts .. you pay your taxes.. may as well make them earn it ...

ken

Every time I go out there, I see something new. I went back out with a magnifying glass. I couldn't find the critter that escaped the web shelter- but @Embothrium it does look a lot like predatory mites from the internet photos I see. @gardengal48 (PNW Z8/9) I'm a little familiar with aphids, and we haven't had issues with them as far as I can tell. Don't know about spider mites. I started to take down some of the webs, and got a better shot of one of spiders which are at the center of each of the thin webs:

Not sure if this helps anyone, but it is totally different from the one beneath the web encampments on leaves. I stopped taking the spider webs because I don't know if they're helping or hurting? Also saw this big insect too:

What I do know is that the thick white webs on the buds/pannicles kill those sections. At a loss of what to do next re: local resources, but @ken_adrian Adrian MI cold Z5 maybe I will start googling bug experts at a county office (?). And what would I be looking for at night as opposed to the day?

Spiders like the one in your picture are predatory so not a threat to your tree. The other is a crane fly so likewise not a problem for the mango.

most Florida mango growers here:

https://tropicalfruitforum.com/index.php

Thanks @Kevin Reilly. I wasn't aware of that forum, it looks great. Maybe I'll pose this question there as well if I can't find more info with local resources. Online sources generally seem to recommend soapy water as a good, benign first step for most mite problems, so I might start there and see how it goes