Well what must have caused it is this is it was in a pot full of garbage from a gum
tree other
weeds etc. were
growing in the pot.
I replanted it in some good cacti mix and a better pot so it's much better off now and getting more sun before it was in the shade a bit too much I think.
It's not looking to bad in the new pot I think and its sure to be much better off now
Here's a picture of my two Barrel
cactus The other one need replanting to, small pot and it was retaining way to much water.
So both of them should grow much better now
Give it the best conditions you can and the new growth should be clean.
Done
(I see no obvious sign of mealies in your pic, check at or below the soil line)
I didn't see anything when I replanted it