While i wouldnt say plecos eating slime coat is common, its certainly not uncommon for it to happen to goldfish. As plecos mature and get large their dietary needs change and particularly fancy goldfish are slow moving and an easy target with a lot of slimecoat to feed on.
Ive never heard of this happening between a BN pleco and a betta before. It probably has infrequently happened and found its way onto the internet where you have read it. BN plecos tend to be small, their dietary needs arent as likely to change as larger species are, while a betta might be slow moving they are small fish and probably not a tempting target for providing a meal of plentiful slime coat. I would say the risk is minimal, but possible if you have found examples of it happening. Keep your pleco properly fed and it wont go looking for other food sources. Let us know how you get on with them and you can pass on your own experience.
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Aiken Drum
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