These penalties are tied into by the Rank 6 ability which you are given by the Legendary Cloak. It's a three minute cooldown that eliminates the slow and damage taken/healing received penalties, so it triggers the Eye and Thing to immediately vanish, and prevents some of them from returning 6 seconds. Ultimately, increasing the status of your Legendary Cloak also raises your Corruption Resistance. This decreases the amount of the Corruption stat you have, without impacting the impact of your Corrupted products. There are Azerite Essences which give a little Corruption Resistance should you equip multiple one of these Essences will provide you Resistance. Allow me to know if you have any questions!
Maintain the necklace. In 8.2 they introduced the Essence system which allows you to power your necklace with a few active and passive skills, and if you are a new player (as in, you don't have them on another character) getting some of these is very very grindy or timelocked. So, unless you're wanting to do end content, in which case you're kinda screwed at this point of the patch, then you can get with some essences rather than the ones to your role.
For advice on resto druid I would check Icy Veins or Wowhead guides on it, and if then you want to go more in depth join your course discord server and feel free to ask questions there and assess out the resources they have. Then gear up a bit with world quests that provide things and attempt to do the 8.3 intro questline which will offer you your mythical cape.
Then, for gearing up, keep doing gear world quests provided that they fall an upgrade for you, same with the emmisaries, and operate a few Mythic 0 dungeons to get loot. Doing some M + keys that are very low is a good way to receive decent-ish loot from a repeatable source. Also the ilevel from M + and raids has although dungeons continue to be something but, thus doing some sort of material will fart equipment astronomically out better than epic dungeons.