It depends on the culture of the university you're at - but if in doubt how to address anyone, you can always ask.

At universities in Australia and New Zealand, an informal "Hello" is probably fine. I know of a few departments where formal titles aren't even on the office doors - just names.

However - if you're feeling awkward because you're unsure whether to be more (or less) formal - why not just ask? Just smile and say something like "I'm actually not sure how to address you because I'm also a student - what would you prefer?".

Most academics I've met prefer informal greetings, and if they've directly given you permission to address them informally, then you don't need to feel awkward about it.