CustomEffectDuration lets you set a custom duration text for Mob Effects.

public class YourMobEffect extends MobEffect implements CustomEffectDuration {
<...>

    @Override
    public Component getDurationText(MobEffectInstance instance) {
        return Component.literal("Hi!!! This is an Example!")
                .withStyle(style -> style.withColor(0xFFFFFF));
    }

}

Instead of showing how long the effect lasts, it will display:

Hi!!! This is an Example!

CustomEffectTooltip lets you add a custom tooltip to Mob Effects.

public class YourMobEffect extends MobEffect implements CustomEffectTooltip {
<...>

    @Override
    public List<Component> getTooltipLines(MobEffectInstance instance) {
        return List.of(
                Component.literal("This Text will appear when you hover over the Effect!")
                        .withStyle(ChatFormatting.WHITE),
                Component.literal("It also supports multiple lines!")
                        .withStyle(ChatFormatting.WHITE)
        );
    }

}

When hovering over the Mob Effect, the custom tooltip text will appear.

CustomEffectTooltip lets you add a custom texture to your MobEffect's background.

public class YourMobEffect extends MobEffect implements StatusEffectBackground {
<...>

    @Override
    public Identifier inventorySprite() {
        return Identifier.fromNamespaceAndPath(YourMod.MOD_ID, "container/inventory/
effect_background");
    }
    @Override
    public Identifier ambientInventorySprite() {
        return Identifier.fromNamespaceAndPath(YourMod.MOD_ID, "container/inventory/
effect_background_ambient");
    }

}

Check https://mcasset.cloud/26.1.2/assets/minecraft/textures/gui/sprites/container/inventory to see how Minecraft does the effect backgrounds. This uses the same logic.