Will a <5mW laser Class IIIa become less dangerous to the eye if I supply it will less current/voltage? Or is it something about the laser itself that makes it dangerous?
In general, the laser hazard depends on the laser power, the output beam diameter, and the laser wavelength. For a class IIIa or 3R laser (the "IIIa" designation is basically obsolete, although it remains in use for products certified before the new classes were defined), you're at low risk if you don't force yourself to stare into the beam. If the beam just happens to stray into your eye, you'll generally have a reflex response to look away from the painfully bright light. (Don't fight this reflex -- keep yourself safe). Note that the high end of the class 3R power limits is defined by the power where 50% of people will have an "aversion response" sufficient to avoid injury --- and the other 50% won't.
Reducing the current to a diode laser will reduce the output power, and thus make the output safer. However, most laser accidents don't happen when the laser is operated as intended or as planned. You also need to consider all the possible "fault conditions" or ways that things can go wrong.
Say you design a control circuit that regulates the laser output to always be less than 1 mW (in most cases, a "safe" level) using a feedback photodiode. For real safety, you should also consider things like
These are the kind of conditions that marketable laser products need to consider before they can pass regulatory requirements. Before you risk your eyesight with your laser system, you should at least consider doing this kind of analysis for yourself.
If you used a laser with less power capability, you would know that before any of these kind of hazards could lead to the laser producing a dangerous beam, the laser itself would burn itself up. If you use a laser capable of producing 5 mW before burning out, you'd be wise to treat it with a proportional level of respect.
There are two main dangers from lasers:
Because of these two factors, the energy density in the middle of the spot can be high enough to burn things, like your retina. To reduce the energy density, reduce the combination of the two items above. Running the laser with less current reduces #2, so yes, that makes it safer.
You would have to explain and understand the basis for your question far more than you have done to make it able to be reasonably answered. As a rough guide, if you ask that sort of question at that degree of simplicity then you re dealing with something outside your safe competence level. That's not meant to be rude - just almost certainly true.
In most cases A LASER tends to work within a relatively restricted range of power outputs. You may be able to control one over a wide range if you know what you are doing but just changing the voltage is usually not going to work.
"Less dangerous" is hard to quantify.