‘Appeal to Authority’, you can probably trust a scientific consensus about a subject in which they are all experts, but you probably shouldn’t trust an individual expert on a topic for which they are not recognized as an expert.
That in itself is the ad hominem fallacy: you need to judge the claim based on its merits, not the merits of the person making the claim.
For example when David Suzuki talks about climate change and people say “well he’s just a biologist, he’s not qualified!” That may be true but it doesn’t invalidate his statements.