Minecraft 1.14 – how detect blocks at some distance from the player

This will be my first question here about some work I would do for a world that I'm creating. Let's jump into the question.

EASY MODE

I would be able to detect if player is in "range" of a specified block (aka mycelium). If the player has a distance of max 1, I need to spawn some particles around the player. For now, I found a tedious way to do this, but it's not covering all the cases. At the end, I post some images that explain the area mycelium should cover and cases when detection must work. ATTENTION: there is no single block of mycelium. Almost all the world will be covered in mycelium!

How I achieve that now?

For every direction, i have a commandblock checking for player, in a clock. For instance, to detect a player on the top of mycelium I used:

execute as @a at @s if block ~ ~-1 ~ minecraft:mycelium run particle minecraft:happy_villager ~ ~ ~ 1 1 1 1 10 force

This will repeat for every variation. So if I want to detect one step forward or backward, I'll do ~1 ~-1 ~ or ~-1 ~-1 ~, moving the X or the Z coordinate.

Making this for mycelium, it takes for now 5 commandblocks in the clock, and it's not covering, for example, the block above, the corners and the block near, since it will add 6 more commandblocks.

So i started experimenting

I know there is a property of objects, called distance. I tried to put @a[distance=..3] or @s[distance=..3], but the command just stops to work.

And now I'm here

Since I lost many of my time searching, and since is all very confusing because of the 1.13/1.12/1.8 versions, I decided to post an official question.

FINAL OBJECTIVE (put all things I wanted to do toghether)

I want to detect if a mycelium block is 1 block away from the player, in every direction. If yes, I would be able to:

1. Play a sound sequence, made with other commandblocks, to that player (only for that player, or in that position)
2. Apply particle effect in that position
3. Poison and hurt the player

Mycelium will cover almost all the world, so I cannot test for only "specific" positions or make markers. EVERY single mycelium block must be detected, and if detected, must do that sequence of actions at least once.