Languages do share a common ancestor. Most of the south Indian languages are evolved from Proto-South-Dravidian, but Tulu separated earlier on, unlike Malalayalam which separated from Tamil only recently.
Therefore Tulu does share a lot of similarities with Malayalam and vice versa. Of course, a casual traveller to Mangalore or someone having a Tulu friend is not going to notice it.
Does it mean a Malayalam speaker can easily understand spoken Tulu? Not really unless you have a keen interest in languages and you are a very keen listener. But it will be easier than a native Hindi speaker trying to do it.
Here is a table that shows the similarities between Malayalam and Tulu letters (Both Vowels and Consonants). If you learn Tulu script you know more than 50% of Malayalam script already, but that doesn’t mean that you can speak fluent Malayalam. Also Tulunadu is culturally similar to Malabar(Especially North Malabar)