Is this old filler? How to remove?

[Nathanlowe25]

Does this look like old filler on the wall?

If so, does it just need some 120 grit sand paper and elbow grease?

I tried to give a quick hand sand earlier but didn’t budge, thinking it may need a mouse sander or to try and chisel off?

 

[conny]

Looks like a poor attempt at filling, but what with is anyones guess.
You may find, if you can scrape the paint off, that it's a two part epoxy mix which will be tougher than normal polyfilla type stuff.
It may be worth investing in a sander if you have a fair bit to do, but, a sander is only as good as the abrasive sheets you use, so get some decent quality ones. Start with a medium and finish with a fine grade. If the medium doesn't shift it you may need to use a coarse grade to break through the surface.

If you try a chisel you may actually chip it out, in which case you can re-fill with a filler of your choice.