Overhead HP bar Surface properties inlet and studs?

Added a set of property modifications regarding the surfaces not being smooth for your script... I noted them with comments

print 'Hello world!'

--Created by XanthicDragon
--Inspired by "Sword Art Online: Burst"

game:GetService("StarterGui"):SetCoreGuiEnabled("Health", false)


local Players = game:GetService("Players")
local Player = Players.LocalPlayer
local Character
repeat
    wait()
    Character = Player.Character
until Character

local Human = Character:WaitForChild("Humanoid")
coroutine.resume(coroutine.create(function ()
    while wait() do
        if Character:findFirstChild("Health") then
            Character.Health.Disabled = true
        end
    end
end))

local red = Instance.new("Part", Character)

red.FrontSurface = "Smooth" --these here
red.BackSurface = "Smooth"
red.LeftSurface = "Smooth"
red.RightSurface = "Smooth"
red.BottomSurface = "Smooth"
red.TopSurface = "Smooth"

red.Material = "Plastic"
red.BrickColor = BrickColor.Red()
red.FormFactor = "Custom"
red.Size = Vector3.new(4, 0.5, 0.5)
red.Anchored = true

local green = Instance.new("Part", Character)

green.FrontSurface = "Smooth" --and these too
green.BackSurface = "Smooth"
green.LeftSurface = "Smooth"
green.RightSurface = "Smooth"
green.BottomSurface = "Smooth"
green.TopSurface = "Smooth"

green.Material = "Plastic"
green.BrickColor = BrickColor.new("Lime green")
green.FormFactor = "Custom"
green.Size = Vector3.new(1, 0.5, 0.5)
green.Anchored = true
local gmesh = Instance.new("BlockMesh", green)
gmesh.Scale = Vector3.new(4, 1.2, 1.2)

local angle = 0
game:GetService("RunService").RenderStepped:connect(function ()
    angle = angle + 3
    local MH = Human.MaxHealth
    local HP = Human.Health
    if MH ~= math.huge then
        if HP < MH then
            Human.Health = Human.Health + 0.05
        end
        green.BrickColor = BrickColor.new("Lime green")
        gmesh.Scale = Vector3.new(((HP/MH)*4)+0.05, 1.05, 1.05)
        gmesh.Offset = Vector3.new((HP/MH)*2, 0, 0)
        red.CFrame = Character.Head.CFrame * CFrame.new(0, 2, 0) * CFrame.Angles(math.rad(angle), 0, 0)
        green.CFrame = red.CFrame * CFrame.new(-2, 0, 0)
    else
        green.BrickColor = BrickColor.Yellow()
        gmesh.Scale = Vector3.new(4.05, 1.05, 1.05)
        gmesh.Offset = Vector3.new(2, 0, 0)
        red.CFrame = Character.Head.CFrame * CFrame.new(0, 2, 0) * CFrame.Angles(math.rad(angle), 0, 0)
        green.CFrame = red.CFrame * CFrame.new(-2, 0, 0)
    end
end)

This is one of the ways you can do this. Alternatively, you can have the bricks already made prior and simply clone them from a folder or lighting or w/e people store stuff in these days.

example:

local red = game.ServerStorage.Folder.red:Clone()
local green = game.ServerStorage.Folder.green:Clone()

of course, the folder isn't necessary but it'd look nicer.

I cant guarantee that the cloned variables won't bug out if you decide to mess with its CFrame and whatnot, but give it a shot if you find the first solution unappealing.