Whats this growing on my acer? Can anyone advise please?

[jaynemarie]

 

[Wires_Guy_wires]

That's lichen.
Some people treat it with vinegar, but you can also manually take it off.

 

[Arie Botanie]

Lichen is unsightly maybe but it doesn't harm your tree in any way.

 

[Gabler]

Lichen is unsightly maybe but it doesn't harm your tree in any way.

You have that backwards. Lichen is gorgeous, but it can interfere with certain aspects of tree growth. I encourage lichen to grow on the trunks of smooth bark trees, where it adds some texture and interest, and it won't harm the health of the tree. However, it might prevent thick, corky bark from growing if it's the type of lichen that could trap moisture against the trunk, and it can inhibit backbudding in the spots where it grows, so I wouldn't allow it to grow where you're planning to chop a branch back in the future.

 

[Wires_Guy_wires]

I like lichen as they're an indicator species of healthy air.
Heavily polluted air breeds only yellow lichen. Very clean air, breeds all kinds of species in a variety of colors.