it depends on how you manage it, there is nothing inherently bad about algae (other than it growing like a weed and looking kind of nasty) so i don't see why pond algae would be so different from regular tank algae, you will of course have to check for any nasties or unwanted visitors that may be hiding in the algae as these will be the sort of things that your newt won't be too happy to see (his stomach might be tho).
id say go for it, if it does turn out to be the sort of thing that you regret it might be a little hard to completely remove but i have never seen algae as a bad thing and a moderate amount of it can even work as a kind of "carpet" for your little friend to walk on and it makes the tank look more natural (in my opinion).