You can grab release him,
but your ideal is to just wall him with ff aerials.
Squirtle has really good starting aerial mobility from a jump meaning that any whiff (usually from a rising aerial) from a reasonable distance Squirtle can cover with his fair or even UAIR (Squirtles like only second to Wario in that regard IIRC).
To be gay, you just never do anything bar retreating ff fairs for 2 minutes. Squirtle generally has less he can do against that then Wario.
Other thing to remember though is that Squirtle has better grab range than you.
OF course there's grab release to tipper fsmash, regrab or down air.
(And every other move Marth has but those 3 above are the only ones you ever really want to do).
Also, don't get grabbed by squirtle either.
And Squirtle ***** Marth at the ledge if you had to dolphin slash and are 100% or above. It's like bye bye stock.
Smashville is probably the best stage to play them on because you get the moving platform to SAVE YOU FROM HOW MUCH SQUIRTLE AND ZARD **** YOU AT THE LEDGE :'( and squirtle can't avoid grab releases ever.