It looks like GSA from the pic. It's hard to magnify it to get a really good look. From what I know, it can really affect slow growing plants easily -- especially the older bottom leaves of an anubias. Increase in phosphates should help treat it. Further, getting nerite snails can help too. They're apparently great at eating GSA, given that their mass isn't too much for the leaf to support the weight. Maybe get a combination of zebra nerite and olive nerites. I've noticed, well at my LFS
, that the olives are smaller than the zebras I've seen. Could have been age or a species thing, not sure? But, in other words, get small and medium ones.