But regardless of
coinhive, there are some safe web miners that don't run malware content, like
coinimp it's script can't be detected as a threat by any of the anti-viruses and ad blockers. The CPU Usage can also be adjustable to a lower percentage They use a slider to set Such CPU threshold in their script, thus not affecting your hardware.
Any crypto-currency miner (which uses resources of the users system) which is being ran without the user being aware, is automatically "malicious" in my opinion. The malicious intent stems from the action of doing things the user is not aware is being done (e.g. eating up their resources, mining crypto-currency), or distracting them with a service which is deceiving because in reality their resources are being used without them being aware to generate income to the authors.
There's also crypto-currency malware which is Win32 based. You download and run -> now you are infected with a crypto-currency miner which will... mine crypto-currency. Eats up your resources.
This doesn't mean that crypto-currency mining is malicious in itself; it depends on how it is used. If the user agree's to allow crypto-currency mining in exchange for free usage of a service, then they are aware of it and it is perfectly genuine (their decision and they don't mind it). Whereas, if you visit a website which starts deploying scripts to mine crypto-currency, or feeds you a download to desktop-based crypto-currency mining without you being aware of any of it (or the download behaving in a deceiving manner to make you think it is something it isn't - which is what a Trojan is), then it is malicious in my opinion.